I regret declining a really good PhD offer. Can I ask if I can accept the offer after all?

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!).

I don't have much experience of US PhD programs, but in the UK it would be quite likely that the place you declined would have been offered to the next candidate almost immediately after you declined it. Again from a UK perspective, it would also give a somewhat unfavorable impression that you declined and then changed your mind. A potential worry for the University making the offer would be that you might change your mind again at a later point.

You don't mention your reasons for declining the offer the first time, I would reflect on these before doing anything further or in haste.

Having said all that, if after reflection you still feel that it was a mistake to decline the offer, I would suggest informally contacting the University where you declined the offer and asking if there is any possibility for you to change your mind. If you don't ask, you'll never know.

You had a reason to decline. Has the reason evaporated? Why are you changing your mind? You should need to have a good response for that, both for yourself as well as to the place you apply to. As a superviser, I had students changing mind back and fro, which I understand - it's a big decision, and not all information may be available. However, keep in mind that your originally offered position, as stated elsewhere, may have been already given to someone else.