I regret declining a really good PhD offer. Can I ask if I can accept the offer after all? I recently declined a really good PhD offer in the US for another offer from an American university and I'm terribly regretting it now.  

I made my decision on the 15th of April and it's past deadline now. I really want to go to the other school and since the 15th of April was the last working day, I'm hoping that they haven't offered my position to any other aspirant yet. I know this is ethically wrong but I feel miserable about my decision and want to contact the university and ask them if they can still accommodate me. 

As far as the other university is concerned I have only accepted their offer through an email and haven't formally been given the final offer letter yet. Should I go ahead with this?
Dude I am an old guy and here is my advice:


Never be shy to ask for things you don't know.
Accept the fact that you are an imperfect being. You made mistakes, and will make mistakes.
Be open to hear others views, and take moment to digest their words to fully understand their perspective.


Therefore it is totally normal to email the university back with full honesty and tell them that you changed your mind.

BUT before you do that, think twice, thrice or even more. You have already done a mistake for being too quick. Don't be too quick again. At least learn.

To me it seems that maybe you are unstable and change your mind too quickly. Let me blame you a bit: before deciding too quickly, why didn't you just email them back and simply ask "I would appreciate some time to think. Could you please let me know the deadline for my response? Thanks."?

Lesson #1 when you are at a point in time where you need to make a choice, usually the best decision to evaluate and perform is the following:


Evaluate if it's possible to postpone the decision (you might need to ask questions to ensure whether it's possible to postpone the decision).
If it's possible to postpone, then postpone it so that you have more time to think and make a better decision.


Of course, some times it is not possible to postpone a decision because time matters. That's why you need to first evaluate the possibility (or even more accurately: the cost associated with postponing it; if the cost is low, and the value from a more reliable decision is high, then dude postpone it!).