OP you need to decide what’s more important to you - relationship security or your concern for her behaviour/past. It doesn’t come across like you are looking for help for her - your Q is what should I do (for you).
1) As a current boyfriend I would say your responsibility is partly to care for her so you should start by encouraging her to try to move on from a traumatic past perhaps by seeking counselling and support, as it’s causing problems for you both. conflicting behaviours in her which she feels she needs to share as a heads up.
2) it doesn’t sound like she’s 100% satisfied with you tbh, but rather you make her feel safe or loveable by treating her nicely compared to others from her past. I’ve seen this many times with my friends and usually they want the relationship because they ‘should’ yet have a deeper urge to keep exploring or acting out from previous relationships.
That said she is young and you probs are too, not many people are in forever relationships at 19 anyway. She has no reason to settle with you In the long run given her unresolved problems she will want to face them at some point and it may not include you. More than anything you seem apprehensive that you’ll be cheated on, which may or may not happen. It’s sort of the last thing to happen in this sequence of events. You need to be concerned with the root here and now