I'm so sorry. I have a special place in my heart for those BS's who are cheated on while they are pregnant. My WS cheated on me during my first two pregnancies, although I didn't find out until my second daughter was about to turn 2. Apparently, it is pretty common time for men to cheat. Not that this makes it any better, just more common
I struggled with PPD after my second daughter and then again after my 3rd (although much less severe). Although it was partly the hormones, the vast majority of it for me was the stress of my marriage. I didn't know my WH was cheating, but he was being a giant asshole and the pregnancy was stressful, and the two together put me over the edge.
My advice for you is to sit quietly and figure out what you need to make this delivery as newborn time as stress free as possible. Your WH has put you in an impossible situation. I hate him for that, and you can too. It is your job to overcome that for the sake of yourself and your baby.
Is MC too stressful for you at this moment? If yes, stop. Immediately. If he is remorseful, he will get into counseling himself and when and if you are up for MC, you can pick it up some time in the future.
Is having your WS in the house too stressful? Kick him out. Set up times for him to come spend time with your older child when you can go out and be by yourself. If he calls to say goodnight, do not speak to him. I picked up my kids and left for a week after Dday because my 4 year old was picking up on my anger and started saying "we hate Daddy, don't we Mommy." I couldn't do that to her. I went to my mother's and he called and said goodnight to the kids, but I didn't speak to him.
Is having your WS at the birth going to be too difficult? Then he is out. No guilt about this. You must take care of you and that baby. Forget about him and feeling sorry for him. Every time you start to feel bad, think about how he knowingly and willingly exposed you and your unborn child to STD's.
Finally, if you are close with your family, best friend, anyone. Please call and reach out to them for support. You need to be surrounded by love right now. If you can afford one, a doula is a wonderful support at the birth.
I'm sending you big hugs mama.