Why lie?!

so about 2mnths ago I was suspicious that my partner was chatting to other women and I had this feeling for a while. I finally decided after asking and getting no where to go thru his phone which came up with multiple pics and videos that had been exchanged with women. I then asked him about and he said yes he had met up with 1 but only to drop some stuff off to her and didn't believe him at all so I decided to msg this girl a month ago and she responded a week ago saying she had no idea about me and the kids and felt guilty (wasn't her fault) she also showed me a msg that he had sent to her a month prior saying im sorry I have a family please don't tell her anything.
We had quite a good conversation and she came out with all the answers I needed to know. They had met up once back in April (I checked my msgs from back then between partner and I and he had a diff story) she also said that they were going to meet up again on a date a month later and it worked out to be the same date as i miscarried.
We had a good/great sex life but during covid our let our hair down nights became an everyday thing for him as i am working i knew but didn't know how bad it got. After confronting him with all the facts he then somehow made it my issue as i didn't have time for him (he was out every night) 😏
He then blamed her saying she was all over him and what was he spos to do 🤔
I told him I needed my space and took off my engagement ring he then started to accuse me of cheating (I am loyal to a fault) and started crying saying he would harm himself, that i dont care about his feelings and it meant nothing.. ao last night I thought i would let my hair down a bit and as we were becoming touchy with each other I couldn't get what happened out of my head, at the start he was cool and gave me my space to breath but this morning he has woken saying last night was a waste of time.I love him we have been through so much together but I dont trust him. He doesn't understand how hurt I am considering when we met he knew that cheating was a big "NO" for me and he agreed
He is constantly looking over my shoulder if I am on my phone and becomes very over the top when he doesn't get the answers that he wants me to say and it makes me nervous.