Are you saying you're no longer sexually attracted to your girlfriend because her more seasoned sexual history is now becoming more apparent and weighing to you?

You know, it is possible that your wants and values have changed since meeting/dating your girlfriend. That's normal and that's ok - especially for someone in his 20s. Don't let anyone tell you that you're wrong for wanting a relationship with someone who's in a similar place emotionally and sexually.

There seems to be a really convoluted relationship with her child's father that you're also involved in. It's not uncommon for that to get old, either.

This just may not be a good situation for you at all, but I only know what you've posted.

I would recommend you talking it out one-on-one with a counselor, but, in reality, you're going to have to share these feelings with her honestly and clearly.