Relationship doomed to failure?

I'm in a long distance relationship, but it's not "seeing each other once a week/month" she lives almost on the other end of fucking north america.

She can't even take a plane trip to see me, she can't because she/ or at least her mother don't have documentation, she has to go down to mexico, get documention, and re enter the US.
She has to be 18 before then, which brings me to my next point.
I'm 18, I thought she was 16, exactly 2 years and 10 days younger.
I was so sure she was 16, I probably even asked to be 100% sure, but turns out she's actually 15.

I thought she told me she was 16, but she's not.

It freaks me out because she's only 3 months older than my little sister and GAWD i can't stand 14 year olds.

Still, she's pretty mature, and I wouldn't hurt her.
In fact, she's got an emotionally abusive mother, she's been sexually abused before, had at most 3 sexual partners, is bullied in school, and has pretty bad depression, as well as being 6 months sober.
She SOUNDS like really bad news, but she's just in a bad place and that's what scares me, if I break up with her she could end up with someone who hurts her, some older guy who won't take no for an answer.

I don't want her to hurt herself, we've both had substance abuse issues, eating disorder issues and depression, I'm scared it wouod trigger her.
Another thing, I'd only be breaking up for selfish reasons, I'm scared the relationship is doomed and it will just unravel and fade away, which is so sad.
I'd worry all the time about how she's doing, and one day I'd know I'm 23, and she's 20 and I broke up with her over something rediculous

She's my first girlfriend, and i want to hold her and make sure she's safe and cheer her up when she's sad but I can't be with her physically.
It's not that I WANT to break up with her, I'm just worried and it might be because they've taken me off my anxiety meds, I just couldn't sleep with this in my head.