But she flat out said she can't stand up for herself and doesn't know why.

She's playing this wounded bird for you to white knight over. She's not a victim here. She wants the attention on some level, and more importantly she doesn't respect you enough to treat you like she should. Even if she did have an issue that she recognized was a problem for her, it doesn't take a whole lot of no's to just avoid situations where that could be a problem. I don't think she sounds mature enough to have the kind of relationship you want to have with her. She 21 years old. She's a fucking adult. She needs to learn to act like one. Her ex isn't the one committed to you. She is.

Ultimately you have to be willing to walk away if you want to have a chance for this to not turn into an ugly break up further down the line. "Let's talk. I've thought a lot about what you said recently... and I can't accept that you just have an inability to say no. You're 21 years old. You're an adult. I care about you a lot, but if I decide to carry this on with you I need to know that you have enough respect for me to both not put yourself in positions like you ended up in and to be faithful to me when they're unavoidable. If you don't think that's something you can agree to whole heartedly then say so now and we'll go our separate ways." That sets the expectation that how she is acting is not ok, that she's being immature, and that she is going to have to make an active decision that she is so deficient as a human being that she cannot commit to the basics of being faithful in a relationship that she wants.

If she does agree that she can step up her game, I'd still recommend being wary... but it's possible that she just needs a verbal slap across the back of the head. If she doesn't full force agree that she can learn to use her words and be an adult, bail so fast she gets windburn from your jet stream.