What do you do if you can't forget the last guy?

if you have feelings for him, let him know. have a mature conversation about it, letting him know that you are serious. if you are best friends, remind him that you can't help your feelings and that if he doesn't feel the same way, you will move on. if things don't work out, you will always find someone else. it may sound cliche, but there are tons of guy just like him that you probably haven't met yet. best of luck. :)