Though I think you answered your own question about why you keep finding yourself back in the same mess, I would still like to point out couple of things.

1) Indeed if you believe that you don’t deserve anything better and think you are a bad girlfriend, then you will automatically seek a relationship where those beliefs are validated.

2) People constantly go back to those who have hurt them. There is a logic to it. If someone has hurt you by saying that you are a bad girlfriend, and you believe it, then you’ll end up going back to that person to seek approval. You’ll hope that the person who brought the pain can also take it away and then everything will be OK again. It is a known manipulation technique to take away the approval from someone so that they will become dependent on your opinion on them.

Those are usual biases and it’s hard to see it when you’re in it. No shame in that, and we’ve probably all fallen for those things in one form or another.