How do you know you've outgrown a friend? Also read below, I need friendship advice!

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I've been realizing more and more I simply don't want to be friends with a girk I've been best friends with my whole life it is our senior year in high school and she's changed a lot and our friendship is very codependent. There's a lot going on in her personal life and I've been with her through it all but my advice is never enough, she constantly talks about herself. She mimmicks me in everything I do, I even applied for my colleges and found out she applied to the same exact ones. She'll often say to me "you'll never leave me right? I have abandonment issues" and it scares me. She once even said that our other friend is abandoning her by going to college in another state! She's had eating disorders and mental health issues and she's going through a lot, but there's only so much I can do. I'm not happy with her anymore. She's so lost. She tries so hard to be "cool" in many ways and it's cringy to see. I need a break. I've been distancing myself for awhile and she gets mad about it but then is super nice to me because she knows I can leave her. It's like she's grabbing onto me and I just want to leave.