I'm so sorry for all that you're going through. For the struggles that were a natural consequence" of your behavior, and now for the actions your gf is taking.
What I can tell you is that you are on the right path now. You may not be able to take back the past or control the actions of others (as in your gf or the joint ex), but you are doing the right thing for you. and I believe you can continue doing that with or without the gf, even though I'm sure it probably seems overwhelming.
You don't deserve to be lied to like that, regardless of what you may have done in the past. If the two of you agreed to a fresh start and putting it behind you, then what she's doing is absolutely wrong.
In my opinion not confronting her isn't an option. At least for me it wouldn't be. Where you guys go from here is up to the two of you, but you can't change what you don't agnolege. This needs to be out in the open. Not just what she's doing and who she's doing it with,but why she's doing it. Is she doing it to get back at you because she is still hurting? is she still interested in trying to fix things and if so, why then is she engaging in this behavior Would be just some of the things I'd be expecting her to answer for.
Best of luck to you. Know that you are not alone and I am here if you need a friend.