Concerning her: If this is the way that she typically handles her relationships, then very likely there's nothing you can do for her, good or bad. Maybe it's a personality disorder and it's making her as miserable as it has made you; or maybe she's an idealist (or even just spoiled) and she's got it in her head that it's acceptable to shop around for an ideal relationship and bail out on any relationship that falls short of the ideal.
If she has been repeating this pattern, then the only thing that's going to wise her up will be time. She'll increasingly notice her friends settling down into LTRs and getting married, she'll notice she's spending more and more time alone, and she'll start thinking back and wondering if missed out on a good guy or two along the way. Then something will click, and she'll start making the changes she needs to make in her life. If that includes seeing a shrink, then so be it.
But those sorts of changes are out of your hands. They have nothing to do with you. They come from inside, and they're prompted more by the passage of time than by anything a guy might say or do to her. So don't worry about what you might have done differently or what you might do to help her in the future. When you see her next, just wish her well. Frankly, you don't have as much influence over others as you may think. It's awfully hard to "fix" people when they themselves don't see the need to be fixed.
Concerning you: Don't take relationships so personally. Everyone comes with their own emotional baggage, and all you can do is hope that you two have a matching set of luggage.
Sometimes you can make a difference when you or the other person is ready to make a life change. For example, when you met this latest girl you were at point where you were dissatisfied with passing liaisons and wanted to build something more long-term. So you're willing to negotiate, undertake some changes in yourself, and consider new ways of relating to girls. But your girlfriend is still solidly set in her avoidant ways and not open to change in the way that she does things. And there's nothing you can do about her and her emotional baggage. In her case, something needs to change inside first. So you accept that it's out of your hands and you move on. You find another girl--one who is like you and looking for a change in the way she does relationships, or one who is already in the mindset of building and negotiating relationships for the long-term.
Realize that for most people these days, it takes a few times to get it right. First people have to get past the dating/experimentation mode and start desiring to build something long-term. They have to desire it enough to work at it and make the necessary changes in themselves. And even once they've reached that point, they may still need to crash and burn a few more times in the future. IOW, even at that point they may be repeatedly seeking flawed relationships and it may take a few times before they finally see the mistake they're making and be willing to undertake the next stage of changes in themselves. You mentioned a couple times that your girlfriend was fragile and damaged by past relationships and that you even considered her fragility a source of attraction. It may take you a couple burns to realize that the fragile ones may not be the best choices for you. It may take a couple crashes before you realize that you can't fix other people; at best you can only learn to live with their baggage too. IOW, you may still have some more changes to make in yourself before you really find a good relationship.
Concerning me: Take my advice with a grain of salt. I'm an INFP, and I'm into self-improvement; according to my set of values it's an acceptable trade-off to lose a relationship but in return gain some insight into myself. So I might be more cavalier about ending a relationship than others.
Still, in my experience love tends to work pretty much the same for most people. It's like learning to ride a bike: You have to crash and burn a few times before you get it right.
Young people tend to go into relationships with a lot of unrealistic feel-good expectations, and it may take a couple times before they're willing to lower their expectations and make some needed changes in their own attitude toward others, IOW to mature a bit. And even after they're willing to work harder at their relationships, they may still have to crash a couple times in order to confront some truths about themselves and take a hard look at what kinds of partners they attract or are attracted to.
So I say: Look at relationships as a process. Don't over-invest on the first one (or the second or third one) so that you won't feel overly distraught when the investment goes down the tubes. Keep it light.
Also, don't kill yourself trying to turn a bad relationship into a good one. You can't fix other people. So focus on your own attitude and stay positive in order to give the relationship a good environment for success (i.e., don't release your frustrations into the relationship and turn the relationship into your private toxic dumping ground). If you can stay positive and your partner responds and seems to be trying to meet you halfway, then work with her. If she isn't, then move on.
Finally, when you think you've found a good relationship, remember that people make promises they can't keep. So be generous--both toward your partner and toward yourself. For example, my current wife has repeatedly promised across the years to be a better housekeeper, but the house is still a shambles (and she won't let me or anyone else clean the stuff up because it's her art stuff and she wants to control how and where it's kept). Her intentions are good, but she's probably not going to deliver. So it's up to me to choose to quit worrying about it, give her a pass on it, and enjoy all the other good things she brings to the marriage. Also, it's a nice bargaining chip when I find I can't deliver on everything that I myself promised her at the start of the relationship.
With time, I think the problem goes away. With time, she gets a little better with the cleaning; and with time we both cease to really care about it anymore anyway. And I think indulging either factor contributes toward improvement in the other factor.
Anyway, I think lessons like these are more useful than any number of noble principles like promising to love someone no matter how much they abuse your trust and goodwill. (I'm not much into noble principles these days.)
The lessons again: Be realistic and lower your expectations about what constitutes a relationship and learn to negotiate and jury-rig an arrangement that works for you instead of shopping around for a ready-made ideal relationship. Instead of trying to "fix" people who are locked in destructive patterns, fix yourself by putting your own best foot forward and then look for someone who responds to that and is willing to work with you and meet you halfway. And when you find a working relationship, be generous--there will be things that both of you promise but can't deliver. Don't beat yourself or each other up over them. Show a little generosity and then let time work its magic.
And if a relationship doesn't work, be generous about that too. No use thinking ill of each other or yourself. Both of you both probably tried your best. The failure probably means that you both still have some things to learn and some changes ahead of you yet before you get it right. It usually takes a few times to get it right. So just take it as a learning experience.