How to fix my marriage?

I need help. I have been with my SO for 6 years, we have a 15 MO daughter, who we love to death, and we have always been a happy couple besides the basics. We are young (22) and we are due with our baby boy in April.

Recently things got rough between us and our relationship, after I had my daughter (whom I breast fed and STILL is breastfeeding) I fell into PP depression. I didn’t recognize myself, I felt ugly, my body was different and it wasnt mine anymore. My sex drive dropped and we NEVER had sex. Maybe 1 every 2 months or more.. He always wanted it, always tried to turn me on. I tried but I just wasnt feeling it most of the time. He would also try in the bed at night but i’d usually have my daughter on my nipple and it just made me so uncomfortable I didnt want to be touched until after I laid her down. He always got annoyed.. we also co-slept and it just never worked out. He always tried to be a gentleman and would rub my feet and try his best besides a few areas he slacked at. Fast forward to the end of September this year, things are still rocky between my SO and I. An old fling/friend from my past happened to hit me up, I responded, and we happened to exchange dirty private pictures. I dont know why I did it, I had never done this or cheated before no matter how often my SO got insecure and assumed the worst, because for a long time he did. He also did not like this guy and always assumed I had this guy blocked. I know I could have deleted the evidence before my SO got home but I didnt. I deserved to be caught. We argued, he was hurt. He cried. I cried. I begged him to stay with me. I tried to explain why I did it but it didn’t matter, he didn’t understand how or why I could give everything away to this guy from our past. He was staying at his moms some days and with us the other days, him and I would still discuss it and it got no where trying to fix things, we would still have sex when we were together and he still said he loved me even though it hurts. When he would leave me he would call me a whore, unloyal, a manipulator. Everything to hurt me, an idiot, he wasn’t in love wasnt happy anymore im not special anymore. I would cry, I didnt eat, I would beg. I handed over all my passwords to social media I had NOTHING to hide. Nothing was worth losing my best friend and breaking apart our family. I wanted to fix things and was willing to do anything. On October 15th, he told me “Give me tonight to think and be alone, i’ll forgive you tomorrow we can work things out” and he blocked me. There was no communication for hours. When he finally got home at 4am he called me. He was drunk, he was making up lies about being in the hospital (meanwhile I had been to the ER twice with bleeding during the pregnancy and stress, had a tear in placenta and things were rough on me going through this) He said he was throwing up blood. I didn’t believe him. He went to bed. I woke up the next morning and drove there to find out that he had driven almost 2 hours out of his way to meet up with a stranger and have sex. It was unprotected, he came inside of her, this was TWICE in the time they were together that night. He also has a fetish with blondes, always liked them and she was too. Everything hurt. We were still together in many ways, he was still sleeping with me unprotected (while pregnant! with tear in placenta!!) How could he plan to go way out of his way for sex and NOT GET PROTECTION! I want to forgive him, we have been trying to fix it but he still fights with me about these pictures saying I gave him everything when HE gave her everything! He claims she said she had her tubes tied so it was fine. She begged for it.. and he gave her what she wanted. I am truly disgusted and I am so sorry for what I’d done but I cant seem to get past what he did. Am I wrong? We do love each other and I fucked up first but we have been only with each other for so long and we made babies and now i’ll never be the last person he went in unprotected. What if she lied? What if she gets pregnant anyways? I am sick. I am livid. Things are definitely not the same but I want my husband.. this would not be easy on us to separate but I cant get that out of my head.