two moments:
1) That moment when you just finished your last lecture of your favorite class with your favorite professor--you know, the professor who has become more than just a lecturer, but is also an adviser and a friend and a teacher--and it feels like a part of you just died as you leave the classroom. It's like finishing a really amazing book with no sequel, but instead of saying goodbye to made-up people, you're saying goodbye to real-life people.
2) That moment when you really want solid advice and someone to tell you what to do because the see-saw of pros and cons is really just going back and forth in your head, but no one is able to tell you what to do because nobody knows the situation in the same way you do. No one knows the people involved or your exact, actual relationship with them. No one can predict the future and all you want is a crystal ball. Or, you know, someone who's been there and can make a really reliable guess.
Bonus third moment: that moment when you're about to give somebody the largest check you've ever written on a bank account full of money you earned yourself in return for a place to live, and you feel really unbelievably old. Not poor, not rich, just old.
On a similar note, someone just asked me where they should send something for me and once upon a time it would have been so simple to just give her your address but now it's just like "Let me think about which one of four residences I might be at when the thing you're sending from god-knows-how-far finally gets here" and then you just give up on thinking and give her your parents' address.