I'm afraid she is just a typical product of an abusive home. Emotionally stunted and unable to cope with things in a healthy balanced way.
This isn't her fault, but nor will it change any time soon. God knows what another two years of living with that kind of emotional abuse will do.
Long term, even if you make it through and she is able to move in with you, she has a lifetime of learned behaviour to undo. It won't disappear because your together,the messiness will intensify cos she will be in a strange place with no one but you to turn to. You would need to ensure she had a job or college to look into so that she could socialise outside the relationship.

Not sure I agree with the previous post, waiting til she is in a good mood to point out the issues might begin to feel like a punishment. Especially since your unsure of her triggers and just what sends her spiralling out of control.

Yes she probably is lashing out at you because your all she has, but guess what, that's not ok. Nor are you expected to sit there and take it.

Your still young yourself and have things of your own to deal with. Your not expected to carry this kind of burden.

You said yourself if her age and mental state had been known to you in the first place you doubt you wld have got involved. This may well become something you resent as the relationship continues, and your anger builds.

I can't tell you what to do, all I know is that the quicker you decide if you love her or just feel sorry for her and obligated, the sooner you can start facing your decision.
Whatever that may be.
All the best, and remember your feelings, needs and wants are valid too.