I think you are lonely, and there is no bigger problem in the world for humans than loneliness. So don't think that you have no good reason to feel so bad. You absolutely do.

The mistake you are making is in thinking that having this teacher is the only opportunity that life ever did, or ever will, give you to not be lonely. It's not. This teacher was so nice and so approachable that she made coming out of your aloneness easy when you were with her. Too bad there weren't lots of people like that around us. Generally, connecting with other people is going to take more work than it did with her. Right now you are not convinced that it's worth the effort. Human beings tend to be economical with their energy. They tend to not put forth effort that they think won't pay off. You have to get around that by forcing yourself to do what it takes to connect with other people. That's hard, but will be worth it.

When you build other relationships, especially one with someone you can be intimate with, this attachment to this teacher will become a pleasant memory, but not an obsession.

Until you accomplished making those connections, you will stay obsessed by what you lost.