How Do I Let Go of a Bad Friend?

So this is a touchy subject for me, I tried to avoid it but it's hard to avoid when I wish it never happened.

A while ago my ex best friend started acting really weird towards me. We've been best friends for almost four years now, and when we first met it was a great experience for me because I was struggling with finding friends and for her to come to me and us becoming instant friends was great! We grew closer and closer and in a matter of months we were best friends. We were inseparable. I even introduced her to her (now ex) boyfriend, they dated for years and she always told me she was eternally grateful for our friendship and how we were like sisters and no one could ever replace me.

Than about a year ago she started acting really differently. We met when we were 16, now I'm 21 and she's almost 21, she started acting out differently than she used to, she started doing things I didn't appreciate and didn't approve of, so she kept secrets from me which we NEVER kept secrets from one another. She started blowing me off more, and stopped talking to me as much, but I couldn't bare to lose her as a friend, the thought of my life without her was hurtful, and I knew I wasn't gonna be able to find a new replacement bestie because no one compares to her crazy ass.

Overtime she got worse, she started ditching me for other friends she had made, because they were able to provide for her more than I could, because I couldn't spoil her like the brat she is, she would rather be with people who have money so she could mooch off of them, and she knows I'd never do that. I earn what I get not to waist it on stuff she is gonna throw away in a week. She backstabs me constantly, and it seems the older she gets the worse she becomes and it's heart breaking, we were so close and it's difficult to let our memories go, but I know if I meet another friend I can make better memories, because why should I have to go through this with someone who doesn't appreciate what I've done for them.

I'm not going to say she was NEVER there for me because she did something for me too, she took me to Tennessee for the first time, she never took anyone and took me because we were still close at the time, but now she never gives me the time of day. I want to get rid of her, get her out of my life, she's like a bad disease, she's there but she causes me misery and pain when she is there. But it's hard to let go because you're so used to it.

So what do I do? How do I let go? Has anyone been through this like me? I know it's not easy, it's probably harder than a breakup. Lovers come and go but friends are supposed to be there until the end, and I was given the ultimate shit friend card, it's time to revoke it..