I'm glad, for starters, that Lexi isn't your friend anymore, she doesn't sound like someone I'd want in my life and truthfully the fact that she is making fun of you is something that really could be reported, she doesn't know it but she's giving you a reason to hurt yourself more, and she can get in serious trouble for that. By the way, I'm sorry you're a cutter, I know how that goes, I've been one for 2 years now...
If you think you and Jamie should stay just friends, then you need to make sure people know that, they're wasting your time and hopefulness by pressuring you to get back together with him. Jamie didn't talk to you for months, but I'm sure over those months he had a lot of time to think, and maybe eventually you guys will find a friendship balanced out in the midst of all of the trouble your friend group is having. He went out with you, so he should know you enough to know that you wouldn't ever hate him, and just want things to be okay.
Make sure Austin knows you're there for him, and maybe his girlfriend is pressuring you to go out with Jamie again because she sees you as a threat? Or is it Austin that's pressuring you? Well, either way, make sure they know that you and Austin are just friends as well, I can clearly see that you care about him if you're concerned about his girlfriend. Maybe he'll start to see that he deserves better?
Finally, I've been through the Chance thing, and that one really sucks. Honestly, if you're "friendzoned", then be that friend to him, get his trust and then somehow talk him into believing that the stalker thing was a total lie. Lexi sounds like a pain in the butt (sorry, I'm an honest person ). And don't let him think that you're not good enough for him, make him see the great person you are, show him that you trust him and that he should trust you
...That's what I'd do anyways

Yeah and the worst part is my EX best friend Lexi knows about me being a cutter and she stillakes fun of me... And she has just ruined everything with my friends... After i broke up with my ex Jamie she convinced him that me and our friends hated him so he didn't talk to us for almost two months! And now my bro Austin is haveing trouble with his minipulative girlfriend who keeps presuring me to get back together with Jamie and i don't want too we are better off as friends... And then i tried asking my friend Chance out and he friendzoned me! And it was because Lexi told him that i had some really weird stalker thing for him! God she makes me so mad!!!!

If your friends are causing you more problems, maybe taking a break from them or sitting them down and straight-out asking them for some comfort or advice... if they care about you, they'll understand That happens a lot with my friends, actually, same thing with "M" not being able to come over, a couple of my friends say that they aren't allowed to talk to me anymore... anyways, its a test of real friendship, if she can't come over anymore but she still is your best friend sister person and loves you, you guys will find a way, maybe hanging out after school at a park or movie, something?
The actual sister thing is pretty efed up, I won't kid you, but you know what, they still love you, I can promise you that. Sometimes its hard to accept something, especially in adults. Sometimes they cling to something they know for certain, like your sister, and they aren't trying to forget you or anything, its a weird kind of favoritism and it isn't fair to you whatsoever! I'm sorry, that really truly sucks, I wish I had something better to say than "it sucks"...

Not so good right now... My friends are causing me more problems then they are at trying to help me with them... Lets see what else... Oh yeah my Lil sis M *best friend like sister* is not allowed to come over to my house anymore because her dad is being an ass... Um... Oh yeah all my parents care about is my real sis which is efed up if you ask me! And so much more...