Girl I like in a bad relationshiip

So I think I have just totally messed things up with her. I told her boyfriend about the messages and pictures and he said he didn't care. Now I really don't know what to do

In what universe did this seem like a good idea?

That aside, everything you've said about this girl so far sounds like a red flag. I'd leave it if I were you.

I don't know really, jealous I guess. And I know yeah I don't know what keeps dragging me back :/

She has treated you like a 'thing on the side'. I'm really sorry about it and I know it's easier said than done but you need to lose contact with you, she is using you and now that you told her boyfriend and that has backfired, I really think it will be in your best interest to ignore her and him. If you could delete her facebook, number, whatever then that would be the first step in doing so but if it is too hard then can I suggest you just keep yourself busy. You could do this by taking up a hobby, hanging around with friends, working, anything so that it takes your mind off of her and the whole situation. You could also take a piece of paper and write down all your thoughts and feelings about the situation and once you're finished burn it and from then on promise yourself not to look back again. I hope things turn out better in the future.

That's part of my problem, I'm sat at home doing nothing waiting to go to uni only working a few days a week. I'm bored and lonely and she might have changed that. Now I feel even worse because I think I've messed everything up :/

Don't you have anyone you could talk to, like some friends? Anyone you can call on to just go out for a bit, that might help you. To be honest, I don't think you could mess something up that was messed up from the start, no offence but what she did to you was out of order and mucked up. :L

Tbh I haven't told any of my friends about her as I haven't met her yet. Now I have her boyfriend texting and ringing me, wish I kept my mouth shut,

What the hell is he texting you and ringing you for. Tell him to suck it up, his girlfriend's a cheat and if he doesn't believe you then so be it. Sure, it may not have been a good idea to tell him in the first place because I wouldn't think he'd believe a complete stranger but what's done is done, block his number. It is obvious that he lied when he said he doesn't care. Sometimes, silence is the best way to deal with situations as silence hurts more than having an argument. Don't feed into it.

Doing my best to keep my mouth shut, he's going on about how they tell eachother everything and that he knows everything we did which is far from the truth. I've said to her I want to speak to her when she's alone. This sucks :/.