Why is my boyfriend acting like this ? (men please give some answers to)?

me and my boyfriend havent been getting on well at all ! all we seem to do is argue so i finished with him for the sake of our 7 week old baby but he came to my house and wouldnt give in till i gave us another chance so i did.but since then when ever we argue he always says he wishes he never met me and doesnt wanna be with me and doesnt no what he see's in me i even found emails from other girls all ending with kisses one of them even emailed me telling me to tell my boyfriend to stop asking her to sleep with him when i said it he completley denied it and showed me his emails to prove it so i believed it but because of this girl emailing me i got paranoid and checked his facebook and emails when he was at work and he obviously didnt realise he had a sent box because all the emails he had sent girls were there and his email account that his facebook notifications go to showed me ll the ones girls had sent there wasnt anything obvious that says he was cheating but most of the emails say come on msn so i cant check what was said on there. but then there are days when he will cuddle me and tell me he loves me so much and he never wants to loose me and wants to marry me. i just really dont get him cause if he wants other girls and doesnt wanna be with me why wont he let me finish with him i hate all the arguing and tension around our baby im just so confused i dont no what to do anymore can some one please give me advice on this