Should I be upset?

So I just found out that my husband had an affair with a married woman for 1.5 years before we got together. He claimed he was unaware she was married, and he stated that he ended it when he found out.

I found where she has sent him pictures of her son, saying it’s his. I asked who the child was and he told me that it was his best friend’s sister baby. I asked why she would be sending him pictures, and he stated then that it was the woman he had an affair with. I asked if he was his child, and he stated that she told him it was but he knew that it wasn’t. He would meet her every week or so to see the boy, but he stated that he just did that so she would stop bugging him everyday.

I asked why he lied about everything, and he told me that he didn’t want me to think less of him.

Now I have a hard time trusting him, even though I never had an issue where I didn’t believe him before this incident.

Am I wrong? Should I let it go or what?