Before hand: I'm sorry about my grammar and spelling, I just typed this without even caring about that.

Background: I hate drama, my sister starts it all the time and never mind I won't start that, my point is I hate drama...yet... I had started some...I hate myself for it but it was eating me up and I had to tell my feelings to this person about it. Her "Morail"(what they call it) aka best friend used to bully my sister and took my mother's pills who needed them for her PTSD. Let's call this person Dan and my friend Jane.

I had warned Jane about Dan and yet she continued to be friends with Dan. Then Dan starts shit at a con and I over hear that Dan has been talking shit about my baby sister. I let it go because I didn't want to start something. But today I annoymisly told Jane how I felt about our friendship how I valued it and yet how she didn't care enough to listen to my advice. Dan had hurt another person and many other people, Jane says she haven't seen this happen and won't believe me.
I have broken my friendship with Jane all though I did have a crush on her.
I hate myself for causing this again but it's hard to get away from both of them when they are both in the same cosplay area as me. I know I shouldn't have done it but I couldn't just leave my friendship with Jane without explaining...

Help: I feel like complete shit for it and I don't know how to fix it or just let it go away over time...

hello
I don't think you should hate yourself for telling her how you really feel. you were being honest and trying to protect her from someone you thought would hurt her and that isn't a bad thing. I think you should just give her some space and possibly try to reconcile with her or re-explain yourself later on. You can't force people to do what you think they should do so you may have to come with terms that she will remain friends with the person you don't like and try to tolerate that guy the best you can.
hope this helped a bit and good luck with your situation
~Kaiyo