My best friend and I have known each other for 15 years. We've been together through thick and thin, but recently he has become toxic and is playing a role in my declining mental health. I've talked to him about it, and he said he's do better to not be toxic, but that was months ago. Nothing has changed. He is apart of our friend group so it's hard to cut him off without potentially saying goodbye to many other friends. I'm still in high school and I don't know what to do.