Why can't I stop thinking about him?

The fact that he's my roomate doesn't help for sure but we don't see each other anymore now (it has been almost 1 month we have been avoiding each other)

It started so perfectly between us but ended not in a good way.

Then I met such a nice guy and we're dating and everything is perfect but...why cant I help being sad sometimes because of the other guy?😢

I'm on second year at Uni and he's a third year and he graduates the day just before my last exam...then he will be gone while Im gonna be studying, I'm sure there will not be a goodbye...nothing...And it's really sad since we used to be really close the first months...i'm gonna lose a friend...once I was crying all the afternoon long by just thinking I will never see him again...

Dont know if I should do nothing at all (also it's not really fair for the guy I'm dating) or if I should do smth but don't know what and it gets so awkward...

Any similar situation you have ever lived? Any advice?🙁