Hi, Memoree

We are all here in the same boat. Our loved ones act one way with us and another with the rest of the world, we get the worst part (right behind their own pain). Nobody would believe how they act at home. Right? We all get from the best, to any nasty name. And none of us is either.
What you describe is what we all go through. It can get better, it does get better. But it is not a straight line.
Therapy can make things worse before it makes them better. Or if it's not the right therapy/therapist, it maybe won't help. We have to root for them to improve, but we can only improve ourselves and our life.
How long have you been together? How long do you know the diagnosis? Have you been reading about it?
Here on this site you can learn how to better comunicate with him and deal with the BPD behaviours, be heard and understood, and find support to take care of yourself in the process. You can start with the basic tools on the right of this page.
Personaly, I would like to know how did you manage to forgive the cheating. Because my GF thinks I've cheated on her, or that I continually do it, and even though I didn't, she can't move past that.
Sadly, it is common, one of the destructive behaviors that they do in order to escape from the pain, is cheating. That allow them to escape from their reality for a moment, even when they know that it will make everything worse after.
I hope you find some peace and a light in the path, by sharing your story and learning about this disorder. We are here to listen.