On Sunday night DP told me he was going to the pub. He didn't leave til 9, and before he left he was trying to do something on his phone and I offered to help. He went all weird and basically snatched the phone away from me and shoved it in his pocket. Then he left.
After about an hour of me getting more and more suspicious, I logged into his Facebook (he had previously told me his password) and found messages to a woman where he had said he was going to her house and that he had "just left" an hour before. I called him. No answer. I left him a message saying what I had seen and that he better answer his phone. Left it ten minutes. Called him again. No answer. Left him a shouty message. Then text him telling him to answer the phone. Called back. Finally he answered.
I told him I knew he was at some womans house, and he denied it, saying he had just left the pub, and kept denying it, even after I said I'd seen the messages and he said he was going to go there but had decided not to and had been walking around for an hour. I didn't believe him so I got off the phone and went on his Facebook and sent a message to the girl asking if "I" could go back. Straightaway got a message saying "Of course you don't need to ask xx"
I called him again and he said he was on his way home. In a fit of rage I pulled all of his clothes out ready to pack up, then decided to read back through all the messages
There were constant messages from Friday to Sunday (the fucking idiot had told her he couldn't text so I wouldn't see the texts 
). He had told her that we were on a break, that he didn't love me and he was living back at his mums. She had said that when she saw him a couple of weeks before (with me!) that she realised how much she still fancied him (they were together four years ago, as i found out later). Then they had arranged for him to go round after her kids were in bed on Saturday, but he cancelled as he was out getting drunk (he was sitting next to me watching tv when he sent those messages). There were messages in there from him saying that "I'm a man with needs. If i want sex, I'll go out and get it" and a couple of flirty messages.
Then she asked why he wanted to meet up with her and he said "to catch up". Her response was "that's partly why i want to see you, and because you're still sexy, and I'm a sucker for your eyes. The fact that I'm horny has absolutely nothing to do with it lol ;-)"
when he got in we had a blazing row where he admitted going round there (obviously) but that it wasn't for sex, they had a cup of tea and talked but that he had already left before i started calling him. He wouldn't let me pack up his stuff and was pleading to stay so he ended up sleeping on the sofa.
The next day my mum took DD out so that we could talk, and we just ended up arguing again as i still didn't believe anything he had said. We had already planned to go out on Monday night, so I agreed to go. Before we left he admitted that the OW had called his phone twice and text him saying she had been waiting around all day for him to call. I got her number and text her telling her that everything he had said was lies, we were still together when he was messaging her. I told her I didn't blame her as she thought he was free. She swore that nothing had happened because he wasn't interested. But did let slip that he was still there when i had called him and had no intention of leaving until that point.
I have no reason to not believe what she said, as she said she had no reason to defend him, he lied to her too.
Tuesday I go snooping again and find a message to a girl a month ago saying that he wished he had ended up with her. And a comment under the same girls picture "Stunning as usual xxx". I know that sounds like nothing, but i think he has told me once in the 2 and a half years we've been together that I so much as looked nice, never mind stunning.
He says he was confused and felt pushed away, and that he doesn't know why he told the first woman that we were on a break (I can only think of one reason a man would do that). He has said sorry, and that he doesn't want to lose me or the baby and that he does love me. He said he carried on lying after I'd caught him because he panicked.
I am sure he hasn't met up with anyone before now as he is a fucking terrible liar and it does seem out of character, but it's fucking ridiculous.
His mum has told him "you're only human, we all make mistakes, move on from it" so now he thinks he's got away with it. Because mummy's word is law.
I, however, think that amount of lies is a fucking monumental mistake.
He's said he will change, and be more affectionate and loving as he hasn't been. And he knows all trust has gone out the window.
I thought I was getting on OK, though the changes don't appear to have started yet. But then tonight, I'm in bed and he's snoring next to me, and everything is whirring around in my mind, and I want to cry, and punch him, and throw him out, and I want a cuddle... Oh I don't know what to do!
I still think that there is something he's not telling me, but I'm not sure what.
He isn't DD's bio dad, but he has been there since i found out I was pregnant (bio dad doesn't want to know, but that's another thread). I don't want DD to have no dad.
Will I feel like this forever if I stay with him? Can I get past it? I have been hurt by every partner I have been with, all bar one cheated, and the bar-one one abused me quite badly. Am I over reacting? Is he sorry? Can he change just that tiny bit? Can our relationship get better?
Please help! I feel like I'm going out of my mind. Sorry about the length 
TIA.