How to stop feeling jealous?

In case any of this matters at all, I'm 15 and I live in the USA. So at this point in my life Ionly really have one friend. She recently got a girlfriend. For a while this friend and I really depended on each other, I guess- we 're both going through a lot of depression and anxiety and all that. And I just.... It's probably wrong of me, but for the longest time it kind of comforted me to know that even thought she was the only person I had, I had her, and that I was the only person she had, too. I know that's wrong of me. I know I should be happy for her if she has other friends. But I've just pretty much spent my whole life bitter because of the fact that I don't have anyone who has ever really cared about me.
Ever since I found out she's seeing someone, I've felt extra awful, like I'm not as important to her anymore, that I'm second. My whole life I've never been anyone's #1 best friend, and it's made me feel like shit. And for the longest time I thought I was most important to her, but obviously I'm not if she's seeing someone, you know? Obviously they're more important to her.
Again, I know all of this sounds so stupid, immature and rude. How do I stop feeling so jealous about her/her girlfriend? and don't think I'm upset because I'm in love with her, I'm not. She's just super important to me.
Maybe people can have friends who are just as important as a romantic partner, is that true?if so I wouldn't know because I've never dated anyone.
So yeah, how can I stop feeling so awful about all of this?