Everyone, regardless of orientation, goes through periods of mild depression for a number of reasons... hormonal, social, crisis, loss, career related....etc. I think that as a young gay man, you're probably going through some isolation or self worth issues and during the times where you think you're looking for bf to be with, you're really looking to be rescued. You're hoping that if you meet this guy, you won't be lonely or bored anymore and things will go on from there. Then you go through a period where maybe you don't feel so vulnerable and during that time, you no longer "need" a guy. These are all just assumptions based on the limited information in your post.
Eitherway, if you EVER feel depressed, talk to a counselor. Sometimes it helps just to have a 3rd party acknowledge your thoughts and help you gain perspective - which is huge. Maybe you can knock it out in 1-2 sessions, or you could choose to go longer. I think the other thing that is super important is finding things to do to occupy your time and thoughts (working out, volunteer, hobbies, join a group/club, bartending, golf lessons). Good luck.