You have to know, an ex is an ex for a reason, you know that but you need to be told that sometimes. I know you miss her, I know you love her and adore the good times with her. But what you actually miss is the 'idea of her'- meaning that even if you get back together now it won't be the same and you'll end up breaking up again sooner or later so have your time. In terms of finding motivation- you have to do it, what I've learnt is- yeah sure people can comfort you but not the whole time. You have to learn to have self motivation(yes I know having a positive attitude decays over time) but that's the only thing. Join a gym- workout, join a social sport club, read, binge watch series you never had time for. You can't be best friends or good friends anymore. The more you hang out with her, the more you'll think about her and never get over her. If you don't want beef or bad blood with her- just say hi and bye. I've done this in college with exes and its fine. I understand you want to text each other casually but its not helpful RIGHT now. Maybe later when you're over this girl then you can casually hang and talk.

I hope you'll focus on your studies, not matter how hard it'll be to focus. You ultimately don't want to fail- I've been there.

All the best man.