Backup friend

I've learned quickly that I don't truly have friends of my own. All of my friends have friends that they like more than me and I'm only here as a backup for when it's convenient. I had this girl I considered my best friend, which is a huge title. We were best friends for 12 years. We watched each other grow up and everything. Last year, she found new friends and made a new friend group and she slowly started replacing me. I have other friends too, or I consider them to be friends at least. Either way, all of the friends I have already have their own friends, so really...I have no one to go to or anywhere to go to. Everyone has someone that they like better than me. I just...exist. Am I not deserving? Am I the problem? I can't be. I do everything for everyone making sure they're happy. Im just pissed off. This shit isn't fair. Why am I so replaceable?