Cheating husband

Its been a tough week for me and i have no one to confide in. The story happens when we were in a relationship for two years and i have a son out of wedlock with him. My husband(then boyfriend)came to know someone whom he worked with at that time and at first it was only just colleague friendship which i don't mind at all since he never did any of this stuff before when he's with me. So after months of me telling him that i'm not okay with them being friends he "claimed" that he ignored her etc. So moving forward,we got married after i give birth to my son and its been 1 year plus. This week,i went thru his ipad,just randomly looking at pictures of us and our kids when suddenly i found a text that at first i find it hard to believe cause yes the way the text read is different from the way i always text him so at first i have my doubts,told myself it couldnt be my husband,maybe it was his friend cause the number is not saved so i asked him and he act all stupid with me. So i asked him again after i put in the number on my phone then i know who it was so i told him not to lie and he told me that he had a relationship with her when i was pregnant with my first son. He told me it was only 2-3 days after that he end things with her. Im not dumb so i told him im calling the girl up to ask her and when i did she told me they had a relationship for months up till i'm about to give birth from around september 2020-november 2020 and they had gotten intimate quite a few times,she send me videos of them together talking after sex and he asked her if it was good and she was smiling and touching him,sends me conversations of them and pictures of them together,kissing and hugging all the while i was at my parents home heavily pregnant and insecure about myself. It kills me seeing him so happy with her. They were also talking about me how she's jealous he was spending almost all of his time with me etc. I broke down cause i was nothing but faithful to him,i trusted him and ignore everyone who told me not to be with him. He insults me throughout my pregnancy claiming i have another man thats why i have a child and my son couldn't be his. All the while he's the one being unfaithful. My heart hurts and the more i look at him the more i think about it. Eventhough it didn't happen when we are married it still hurts and when i think about it he did act suspicious throughout our relationship and i kind of knew about their relationship but i didnt want to believe it cause i trust and love him. What hurt me the most is that why must i found out now that we are married,why not before i give birth?i look at my son cause he came to me when i was crying,holding me and trying to comfort me eventhough he dont understand anything and i feel guilty. Him and i we went thru alot during my pregnancy with him. I cried alot thats why i think he had that bond with me. I decided to stay although i dont know if thats a good decision for me now that i have two kids with fear he would do it again. He told me he wont but i dont know...its just mentally f***** me up.