Your both young and have a past with other people..

I can understand your concerns and being worried because he did "in the past" kiss a girl.. You said you eventually dismissed it? Well you didn't, not 100% as your mind flips back to that any time you get insecure.. And hey its okay I think most everyone has felt and done the same, really. But it has to change.

How to move forward and stop the constant worry about him being loyal and faithful ? You and him need to sit down and have a talk about your relationship what you want and need and what you can't and won't.. It's really not fair to hold that kiss over his head, that's just going to cause resentment on his part and rightfully so, if tables were turned you would feel the same.

Since all this is actually causing you physical problems you need to just get over the past incident, I mean he knows he did wrong, time to put that outta your mind... You are going to have to trust him 100% not 99% all the way or nothing. How is your relationship ever going to work and grow if you can't trust and believe in him ?

Everyone has fears of being hurt by someone you love.. But if your always worried about what "might" happen, you are going to miss all the good things you have with him.

This trust is a two way street , he is also going to have to trust you while your away at University.

If you just can't let it go and move forward, I would advise you to talk to a therapist about how to move forward.

You have to have trust in a relationship, if you don't that relationship is doomed.

Good luck and Welcome to PC