Boyfriend has changed since having our 4 month old baby?

since having our daughter he has joined the gym which we argued about because it was more money to pay out and I am not working at the moment but he still went ahead and joined it. I said our daughter is going to cost more with food and nappies as she gets older. He just thinks of himself. We have had problems with money in the past because he is useless with it. He goes there every night now for hour or two after I have been at home all day looking after little one. I have to ask him to help with nappy changing when hes here. When he is home hes on facebook or looking at which upsets me. He broke off our engagement last month because I do not trust him very much because of past problems in our relationship where he has gone and got weed and denied it then I have found it (he still is smoking it occasionally when said he would give it up), he has also joined single websites behind my back but nothing proceeded with it as far as I know. Since then he has met up with a girl from work played badminton and gone for a few drinks with her on a weekend spending all day and evening out not telling me this switching his phone off. I only found out cos he was being suspicious with his mobile always in his pocket never out on the table like it used to be, apparently was speaking to her about me and him, he also met up with another girl on a weekend again and would not answer his phone who appears to be an old friend he met on facebook. I said I cant be with him anymore he never wants to do anything with us and seems to be wanting to spend time away from home. Ive had enough, since the engagement broke off its just gone down hill and I am forever thinking what is he up to. That is no way to live is it? Am I being selfish or is it my hormones. We were both so exited for our daughter to arrive but its just so not good anymore we are like strangers and just argue constantly and I feel like I hate it. He said hes moving out then says he does not want to. I think its because he cant afford to coz he has debt and we do not have a mortgage. I do not want to bring our child up with tension in the house its not fair on her she is my main main concern.