Confused and and need help

Hi I'm going to apologize in advance my spelling grammar are shocking. The past 2 weeks my partner & I have been on a rocky path, we have been together for 5 years I recently found out that he had a girl at his work that liked him. He did tell me this but did not go into much detail. So I assumed that he would deal with it in a way that was appropriate without jeopardize our relationship. I did go snooping through his Facebook one night as I was found comments & had been messages to each other, & had both been flirting with each other. But it had only gone on for two days from his side then he put a stop it & told her that he had a family & didn't want to lose them. I had found out that she had been texting him while at work. She had told him that she was in love with him & that she had missed him but he only replied back as I miss you to (I asked him in what context after we fought he said as in friends having lunch) I had asked him if anything more had been going on & if he had gone out with her as she had sent him messages asking him if she could drive him to work .he said No to her & she had asked him to go bowling & other stuff. After that we had a massive fight. Now the thing that pissed me off the most was he had deleted these messages from his Facebook & phone thinking that I had not seen them or didn't know what was going on. & I kept quiet hopping that he would tell me what had been going on. But didn't say a word to me. So I brought it up & asked him what had been going on all he said was there just friends nothing more I said to him that I had seen the messages & I knew what had been said. A few days after he let me into his account on Facebook & I sent her a message. Asking her side of things she knew it was me talking to her as my partner watched the conversation take place. Before I spoke to her I asked him if there was anything that he needed to come clean about as if I find out something more that he has not told me that I would be mad. He replied with no I have told you everything. I then find out from her that he has spoken to her about our sex life & how we haven't done anything in months & had complained about me not doing anything on top of that I had found out from her that he had gone out with her twice. While I was at work to the park and that she had picked him up but apparently all they did was talk. I had asked him if there was any sexual contact between them he has said no & so did she. (but when I found out they had gone to the park together while I was at work I snapped I was so hurt and betrayed & felt cheated by him the trust I did have for him was gone because he had lied to me again. Telling me he didn't go out and do anything with her when he did after that we broke up I felt so hurt & betrayed by the one person I thought that would not hurt or lie, Did just that to me.His mother got involved so I told her what had gone on plus her son had told his side of the story to her. All she had to say was that he had not cheated on me & he did tell her that he had a family. & that I was over overreacting over the whole situation. I tried to explain to her that it wasn't so much the fact that it was about this girl. But for me it was more the fact that he had lied & hid & delete the messages & did go out with her and lie. Then didn't say anything to me about the whole thing. I did keep in the back of my mind that her opinion was bias as it was her son. so of course the favour would fall on his side plus anything I said his mother would find some excuse for his actions & then say that I shouldn't of been snooping around anyway that I have trust issues (I do) But every question I asked him or I try & explain to him that it was still feel like cheating to me as he lied & was deceiving me. All he kept saying was were just friends nothing else & that he didn't want to lose me or my daughter. He explained that it had got out of hand that he thought that he could of handle the situation on his own & he didn't want me to find out as he knew I would fly off my handle at him. (I did fly off the handle at him more when I found out) We did not speak for 4 days and I left the house. We now both have spoken & have been trying to work things out. I do know that he dose love me & my daughter very much. He has now removed her off Facebook & has blocked her off his account. & has let work know he does not want to be around her. He found out at work that she has done this to a number of guys at work & have jeopardize their relationships with their partners... when he went back to work he did tell me that she came up to him & asked if they could still be friends But he told her no. Now that we are trying to work this out I am finding it hard to to trust him or believe anything he tells me. And every time I'm at work I’m thinking that he is doing something behind my back. How can I regain trust back with him as I know if I can't it’s not going to work? Please help me...

Sorry to hear bout this im in same situ but worse my girl of 15 years doesnt love me any more and just found out she has been seeing another guy behind my back. Its been 2 months since spit im back at parents she has our family house the kids and car i get to see kids on weekends while this guy is making my ex fall in love eith him im totally broken knowing there isnt any chance of getter back. My advise is dont do what i am goin through the longer you are tgether the more hurt u will become . But then if u can find it in jour heart to really trust him try to make it work but be carefull once a cheat alwaycs a cheat even though i wud take my ex back even if i wasnt happy for the sake of kids though.