I agree it can be hard, specially if you feel lonely. The thing is most people don't like to be alone, don't know how to be alone, just like you said. Not wanting to bring that "oh, things this generation do" here, but most of my peers who seem to have this very same kind of anxiety you're talking about are extremely active in social media like facebook, instagram. These people don't know how to be alone, they are constantly seeking validation and affirmation through social media, text 24hs/day, can't stand a weekend at home. A girl I know from college said she was using tinder because she had nothing better to do on a saturday.

What are you studying? I'm in college and, even though I have a relationship, it takes like 2/3 of my time. You should try to keep yourself busy, sign up for a class you've always wanted, specially if you live close to a big center or to your Uni. Free courses maybe, since you're a student, idk. Aren't you also overthinking a lot about that relationship that ended bad? This probably has a lot to do with you not coping with it too, as you said those were your happiest moments.