Jealousy I can't control.. Advice ?

So I basically have a problem I can't handle and wanted to ask if someone might have advice.

It's not like I don't have friends, I just don't easily consider people I hang a lot around with friends. However whenever these people meet up with other ones I always feel so lonely like no one cared about me. I can't control that jealousy, I do have people who care about me, but it drives me mad either way. Does someone have a tip how I could improve myself?