Twice bitten!

okay so I had to do something, I have been going mental for years but its hard to know who to ask for advice! I have been with my partner for around 5 years and I am currently pregnant with our 3rd child. around 2 years into our relationship, through the wonders of facebook I found out he had cheated with a woman he worked with. he claimed there were no feelings involved and begged forgivness. there had been a lot of strain on our relationship as I had been suffering from depression and had neglected him somewhat. I made the decision to forgive him and we moved forward with our relationship. I thought we were stronger than ever until about 10 months later when he walked out on me. after a day or so he told me the reason he had left was because he had cheated on me again with a different woman from work and couldn't handle the guilt. now call me an idiot but other than the obvious he is a really good man, so I forgave him again! now the problem - he has secret, deleted conversations with a third womanhe promises me that nothing ever has or will happen and that they are just friends but I don't believe him! I once found a message I recovered in his emails saying the bitch was onto me so I had to keep my distance for abit!! sometimes we have conversations and he will go months without speaking to her but it seems as soon as we hit a rough patch he turns to her. the thing is I found a message this morning as he is at work and hasn't seen it to delete it. its from last night and said, "I thought you were coming round tonight????" am I a fool to keep believing in him? we don't have much intimacy in our relationship which is down to me and I just feel like he may be getting it else where! it is driving me crazy, I even asked the woman he speaks to, not to but she apparently has no respect for me either! any help would be very much appreciated!