He cheated during my pregnancy and lied for over 12mths, now what?

Hi, excuse me if I waffle on a bit, I haven't posted anywhere before but things are coming to a head and I need some help or at least to know I'm not loosing it completely.... Here goes.... About 2 years ago (7mths pregnant at the time) my Husband got a text message on his phone that said "are you ignoring me?" I wouldn't have thought anything of it, except his reaction made me, he panicked and snatched the phone away so I asked what was going on, the guilt was so clear all over his face and I felt my heart break but he denied anything and said that a lady at work had been pestering him via e-mail and text etc. So I asked to see his computer, he threw it and smashed it. He then changed his story and said a different work colleague had been flirting with him and he didn't know what to do, he went in to great detail about how she was coming on to him and he felt awkward etc. I never bought in to that but I was so heavily pregnant I had more important things to concentrate on. After a while it was playing on my mind so I kept on asking him, that then turned in to begging him for the truth, he kept on saying I was paranoid and needed professional help and was making his life hell. Four months later he told me he had been lying and he actually instigated flirting with said colleague as he felt left out with me having our baby, don't ask me why but I just didn't buy that either. So months went by arguing and living through complete hell, I feel so much love for our daughter and looking at her just makes me feel so sad that we're not happy together, I want so much to give her that stable upbringing every child thrives through. Anyway just about 12 months ago, a year after it happened he cracked and told me he had slept with her when I was just over 7months pregnant, for no reason other than she was a challenge as she was married and trying for a baby with her husband. However the problem I have is he lied to me for so long and changed his story so many times how do I know what he says now is true? Did he have a full blown affair with this lady? The reason I think this is because he used to have to take pills to keep 'firm' he doesn't anymore it was purely a lack of confidence that gave him problems and the pills replaced the problem, however I did monitor them when I thought there was something going on, he kept a pack hidden in the garage and they did go down in number and we weren't having sex at the time or very little due to a very uncomfortable pregnancy. He also had nights away with work often that have now stopped. I just don't understand he is a very attractive man and I don't think I'm bad looking (sorry if that sounds arrogant) but she is not attractive in the slightest, I'm not just saying that because of what has gone on but I laughed when I saw her and please dont think that's a nasty comment I was just so shocked. I feel like I'm loosing grip on my life and I look at our little girl and she is the happiest and most smiley baby I've ever known. I feel like I'm failing her if I give up on our marriage, I love my husband which is why this hurts so bad, I've never felt so alone and desperate. I need to know the truth but I'm not sure if even that would save our relationship now. Honest opinions would really help me if anyone has been in a similar situation I really don't know what to do, we said we'd go to relate, weve got the paperwork but never done anything about it, it's like deep down both of us have given up already.