Q:

How do I tell my new boyfriend that I make a lot of money?

I am usually very secretive about my income. No one really knows how much I make and I was taught to never talk about money to friends. I make about ten times what my boyfriend does. He's also a professional and makes good money himself. We've discussed money before but I've always just avoided the subject. 
He often offers to pay for things and I try to go halfsies with him because I feel bad, but he sometimes insists because he's the guy. 
We've been dating for three months now and I don't like hiding things from him. Like this week I did contract renegotiations with clients and it was stressing me out and it went so well and I couldn't explain all the details why I just had to be really vague.
My biggest fear is that this will change things.

What if he treats me differently?  
What if he doesn't like that I earn more than he does?  "Guys must look after girls" and all that.

I know that I'm probably over reacting to this and it'll probably be fine but I am really stressed about telling him about this and how it will go and if it's the right time. I don't want to make a big deal of it either, I just want him to know and then we go back to normal. How do I do that?
We're in our 20s, in the United Kingdom.

A:

As someone who has been the boyfriend in this scenario, tell him. I can completely understand your concerns but you need to find out sooner rather than later. 

What if he treats me differently?

He might, and you need to know as soon as possible. In my situation the person had the same career as myself but had some spectacularly lucky (wise?) investments so I really had no indicator of the difference in our earnings until she told me. I was relieved. Some of her spending decisions prior to telling me seemed risky at best, after I understood her situation better they made a lot more sense. The good thing here was that she told me about it quite early on, it didn't feel like she had been keeping a secret, it was just something that hadn't come up. If it was two years in before she told me then I think I would've felt hurt. Not only this but let's say he reacts poorly, is jealous or emasculated by your success. Do you really want to find out after you've invested more time into the relationship?

What if he doesn't like that I earn more than he does? "Guys must look after girls" and all that.

I'll be honest here, and I realize it doesn't reflect well on me. I'm the "Guys must look after girls" type of guy. Pulling chairs out, paying for dinner, and all that other slightly sexist although hopefully slightly chivalrous stuff that is frowned upon these days. This didn't change after I knew the difference in our earnings. I never paid for dinner because I thought she couldn't afford it, I paid because I liked her and I'm maybe not good at directly discussing my feelings so: 
"here is some food, I hope you get that this means I like your company". As to this part:

What if he doesn't like that I earn more than he does

Good, you found out early that he's a boy, go and find a man for the time being. On the other (and more likely) hand he probably won't particularly mind, He didn't think you were on the edges of poverty before you started dating. 
A couple of things I would note:

There are some people who would dislike you simply because you earn more than a certain amount, I imagine you would already know if he held that view but it is perhaps worth considering.
Letting him "in on a secret" might be a good way of telling him. This way you're showing trust in him, and can explain why you don't want other people to know.
trust between two people takes time and effort. Giving him more trust will make it easier for him to reciprocate. Having shared secrets can be fun.

This was longer than I intended and a bit meandering.
Tl;dr: Tell him, waiting holds no benefit.

A:

If he's the type of person who will treat you differently once he knows, it might be good to know this sooner rather than later.
You say,

He often offers to pay for things and I try to go halfsies with him because I feel bad, but he sometimes insists because he's the guy. [emphasis mine]

I suggest that you take him out to dinner. Nowhere above his pay grade but someplace nice.
If he is insulted, do you want to continue this relationship?
If he is pleasantly surprised, and you want to continue this relationship, tell him after the dinner that you can easily afford it. Until you're ready to make a full commitment, you need not tell him more. If he insists, you can either tell him (I would) or say politely, "It doesn't matter." His reaction to those things will also inform your decisions about your future.

A:

I am usually very secretive about my income. No one really knows how much I make and I was taught to never talk about money to friends. I make about ten times what my boyfriend does. He's also a professional and makes good money himself. We've discussed money before but I've always just avoided the subject.

This is awesome. Perfect! This works well for friends, family, boy/girl-friends, weirdos, and strangers.  Unfortunately, if you're going to share your life with this person then, well, the income is part of your life. 
Alright here we go. Totally double standard activate!

He often offers to pay for things and I try to go halfsies with him because I feel bad, but he sometimes insists because he's the guy.

Let him. Let him buy. Right or wrong us guys were taught that we should be able to provide. Doesn't mean that you need him to survive financially, but he will still feel that he should be able to provide. Again, it's not about having to provide; it's about ability to provide. Heck I still feel the urge to whip out my debit card when my wife and I are out, and it's a joint account.

We've been dating for three months now and I don't like hiding things from him. Like this week I did contract renegotiations with clients and it was stressing me out and it went so well and I couldn't explain all the details why I just had to be really vague.

RED ALERT!! RED ALERT!!
Why have a boy friend if you can't share your feelings. If it's just for sex, then there are names for that, but it's not boyfriend. If you can't share your feelings then you're in trouble. Yes three months is still new, and yes you have to let a little crazy out at a time, but you should not be in a position where you have to hide feelings. There are so many more fun things to hide. 

My biggest fear is that this will change things.

Change is constant. Get over it. I know that's not nice, but it will change things. It may be for the good; it may be for the worse. But change is change.

What if he treats me differently?

If you like the new treatment, then look at dresses and cakes, if you don't like the new treatment then look at new boyfriends.  Simple. He is not the only man on the planet. There are others. 

What if he doesn't like that I earn more than he does? "Guys must look after girls" and all that.

What a guy wants
It really makes me surprised how many girls don't know what a guy wants. We want essentially 3 things. 

Sex - That's easy.
Emotional Balance - Shhhh it's a secret, but we have feelings too.
To be needed. This is the one you risk toying with. She doesn't need me, she can buy her own happy meal. Well most guys know there are more than a few ways to be needed. So you don't need him to be able to afford your cup-o-noodles. Make him feel needed in other areas. Pick one. Try to keep it simple. "You're the best handy man, best problem fixer, you keep me sane and stable, that thing you do in the bedroom is amazing, You order the best pizza." It doesn't matter. If money is the only way he can feel needed, then it's time to move on. 

Side Note
When we choose who we date, most of us guys choose women that we know can take care of themselves. We still want to be the take-er-care-er-of-er, but how many times have you heard a guy go, "Wow I want to date that homeless chick, she has big knockers and could really put a strain on my wallet!"
Side STORY
I make 5 times what my wife does. She makes awesome money as well. Guess what happened. Sure as water is wet, my business hit a snag and we lived off her income for 6 months. 
Now we are stronger for it, but back when we were dating, we discovered that she was better at getting the deals, I was better at doing the negotiating. 
We looked at aspects of "money" and picked what we were stronger at, so that the "us unit" could be stronger at all aspects of money.

I know that I'm probably over reacting to this and it'll probably be fine but I am really stressed about telling him about this and how it will go and if it's the right time. I don't want to make a big deal of it either, I just want him to know and then we go back to normal. How do I do that?

Then just drop the bomb. 

"Lets go out and celebrate." 
Pick a place that you would normally go, one that is in his price range so he doesn't fudge his pants trying to figure out how to deal with it. 
"I'm so excited. I finally closed the Publeys deal. Man that signing bonus is going to be nice. I mean 50k in one day. I usually have to work at least a week to get that."
Expect a bit of a jaw drop. 
Let him pick up the tab for dinner.
See what happens.

Obligatory warning
When I talk about guys I mean most, normal, good, men. Not the rotten ones. There are plenty of bad men.  Also keep in mind that the notion of good and bad is based in culture.  Though I feel that the info here is generic enough to count.

