am i wrong? (step mom)

My partner (33) male and I (32 female) have been together for about 17 years. We did split up for a few months about 7 years ago, where he met this woman, 6 years younger than him and from what he said, weren’t a relationship, simply just messing around. She was very toxic towards me from the beginning, accusing me of sending her threatening messages and bothering her, which were all a lie. She did work at a boost mobile phone retail store so I’m positive she had access to sending messages to herself from different numbers. Anyways, he ended up telling her that he didn’t want to mess around anymore since apparently, she was trying to control him from co parenting with me (we had two children at the time). Well, a week or two after they stopped messing with each other, we talked about getting back. I was hesitant at first, but I still had feelings for him, and I wanted to give my kids a complete family again. Weeks later, I found out she was pregnant by him. It broke me. I felt like my kids were let down. I did think things through and decided to stay with him. I did have my doubts on whether her baby was my partner’s baby or not but after going through court, it was determined that baby is 100% his baby. The first two years were rough because she pretty much didn’t want me around the baby because she thought I was going to hurt him. I got it. but I would never do such thing. Things are better now and his child is very close to my youngest (I had a third child, 8 months after his baby was born). I have always tried for his child to feel included. But a lot of the times, I prefer not to be around. It’s something in me that makes me very sad and I do get ptsd, I think that’s what I feel like. I don’t treat his child differently than my kids and I would always care for him and defend him like my own. I never my partner he can’t see his child or buy him things because I wouldn’t have wanted any woman to try to stop him from doing that for my kids if we had remained split up. But I just don’t want to be around him as much. Am I wrong for feeling this way?