Q:

Excessive checking and anxiety as I solve lengthy problems or problems with intensive calculations

I suffer from Obsessive Compulsive Disorder. But the good thing is that I managed it 8 months back after therapy and practicing mindfulness meditation on a daily basis. Since then, I have been working on my problem solving skills and learning skills. I have improved. 
In lengthy problems, I often have this 'Something is wrong feeling' which is pretty common for people with OCD (maybe normal people too). I tend to get anxiety because of this and resolve to checking multiple times. This slows me down and often leads to less scores. 
What I think I should do?

Practice more. While practicing, think about each step and do not check more than once. Match final answer if solution is available.
Maybe, try to improve my working memory by doing more mental maths.

Please suggest how can I speed up.

A:

(It goes without saying that this is not actual medical advice. I'm only sharing my thoughts because this is a unique topic I can relate to.)
I have OCD as well, and it occasionally affects how I do math. I'm not sure there is any solution I (or anyone else) can offer, but it definitely helps to identify OCD as the problem. Being able to "other" this condition is useful to me, so that I can at least push back against my own brain. My best advice is the following

practice mindfulness. Try to recognize when you are no longer doing actual math, but really just repeating a ritual.
accept some imperfection. It not feasible to be 100% rigorous when doing math by hand, so recognize that you can allow yourself some leeway.
take reasonable shortcuts. People never say this out loud because most students will take this suggestion and run with it. For those of us with obsessive thoughts, however, it is possible to get too caught up in being 100% precise and rigorous 100% of the time. You are allowed to be somewhat conversational while doing math.
Don't play along with your own brain, if you can help it. OCD is not an urge that can ever be satisfied. Luckily, it can be controlled.

More concretely, here are some things that are important for me.

I avoid doing math right before bed, because I will have racing thoughts about nonsensical math all night. You probably are different, but my point is that you should forcibly cut yourself off if you need to.
Verbalize your progress to someone (or write it down for yourself). This can help pull you back into reality.
Force yourself to put a problem down. This might sound silly, but it is really important to pull yourself out of the guilt cycle.

Anyway, sorry if that was vague. Like I said, this is not really medical advice so much as my personal thoughts on the matter. For what it's worth, you are definitely not alone in the math community in suffering from OCD. I hope it doesn't get in your way too much.

