HI, ALEX! Welcome to the forums. Sounds like you are going through some emotional "unfinished business." I have a lot of that myself. When somebody puts me down, or when I have invested more in a relationship than the other person, or when somebody says something unkind to me and I just ignore it, the situation tends to hang around and haunt me. That's my "bruised ego" talking to me.
TO ACKNOWLEDGE YOU TOOK A LOSS: You probably need closure. By that I mean a way to close the door on the relationship and acknowledge that you were disappointed and gave more than you received. You don't need to see the person or have any contact with her. You can do this all by yourself. First you could acknowledge that it is a matter of hurt ego on your part --- because you sort of know that if the relationship had gone on for a very, very long time --- and you had ended it --- that you wouldn't be hurting. So you kind of feel that you took a loss. Well, you did. That happens to all of us at one time or another. And it's not strange at all that you'd have a little depression about it. Once you understand what's going on, though, you can restore the balance in your own mind. It doesn't mean you're less of a person because it didn't work out. Every relationship is a little unbalanced --- one having more feelings than the other. That's life.
THE IMPORTANT THING IS TO HAVE NO RESENTMENT, BECAUSE IT'S PART OF MATURITY TO ACCEPT LOSSES GRACEFULLY

AND REALIZE THAT WE ALL FACE THEM SOONER OR LATER.
TO OBTAIN CLOSURE --- Some people like to play the couple's favorite song or go to a place that they especially enjoyed --- feeling good thoughts while remembering the good parts of the relationship. Then they express the disappointment and sadness they felt when the relationship ended. Then they say "goodbye" to the person (who is not there) and wish that person well, thanking her for the happiness they had for the time they were together.
ANOTHER WAY TO OBTAIN CLOSURE --- Some people write a letter to the person, expressing their disappointment and sadness about the relationship being over, but also recalling the happy times they had, and saying goodbye. THEN THEY BURN THE LETTER!
Whatever method feels right to you would probably be a good plan for helping you attain closure. There are things you need to feel and things you need to say. Be good to yourself and allow that to happen.
These are only my opinions. But I kind of like them.

Again, welcome to the forums! Good to meet you. Keep talking to us, Alex. We're a rather nice, supportive group.
Adieu