Hi Gavin, I'm sorry you've been through this.
Problem is, this girl is broken - probably due to the trauma she suffered as a child. And because she's broken, she will make unwise choices. Unwise choices such as bad partners. You see, we can't expect for normal behaviour from someone who's still suffering trauma.
I agree that she probably did use you. But I'm not sure she did it consciously....instead, I think you were a soft place to fall and she enjoyed having you around. But when it comes to choosing a boyfriend, her trauma background is making her do terrible choices.
So, where to from here? This girl needs therapy. If she's already doing therapy, then she needs more. But this is far beyond what you are able to do to help her. I'm not quite sure how much contact you have with her now, but I suggest No Contact - if you're not already doing it. You need to give yourself space to get over her. And she needs to accept that she's been stringing you along and that you need space.
good luck and stay strong. You sound like a really great guy - please don't sell yourself short.
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.