Who does one deal with Jealousy?

Doesn't it hurt when you see your crush going out with a group of friends & you have to make sure that they aren't dating any of the guys?
Whenever I am not involved in an outdoor activity with friends, I always feel like they'll forget about me & are okay without me. And this really kinda always spoils my mood for the day.
One instance just happened right now, I was having a perfectly normal day & then I decided to call a girl I like(We study in same college) to ask her if she had paid her college fees, but I heard my senior(A good friend of mine) speaking next to her with a couple of more people(People I consider "Cool") and a rush of depression just hit me. The thoughts of why I wasn't there/does she like hanging out with my senior over me/I also wanna be friends with the other people.
So yea my day has kind of been spoiled by my mind. I really hate it and I try but the depression won't go away easy.
I should be having a normal day with a happy content mood but here I am posting this because I am in pain

 

Look that's not the problem,
Assume she's my girlfriend in the above situation. I would still feel the same things

 

Dude the girl is not the problem, imagine the above situation is a close friend instead