Husband and I are divorcing, living together, hes taking my child around his g/f.

Hello, I've been reading over this site for a couple of months now and just decided to join and finally ask a question of my own.  Let me give you some insight to the situation first..

In July, the day before our 4 yr. anniversary my husband told me he didn't want to be with me anymore, wasn't happy, hadn't loved me for a while.. possibly a year.  He has given me many different reasons why, I'm jealous and can't get over the fact that he has a past, I worry and stress about money too much.. thus making it seem like I saw him as only a paycheck, etc. etc.  We were together for 6 1/2 yrs, married for 4.  We have a 3 1/2 yr old daughter.  During my fight to save our marriage I asked why he was fighting against it so bad, was there someone he wanted to talk to?  He then said yes, [name].  I knew he was talking with her, he told me it was for advice about our relationship from someone outside of our circle of friends.  He swears up and down that shes not the reason hes done this, he was leaving me before he started talking to her. [They grew up together, dated once or twice 10 yrs ago.. fell for eachother, went their seperate ways, found eachother online about a year ago]  I didn't see any of this coming... I still think that if shes not the whole reason this is happening, shes takes part in it some how.  He fights tooth and nail that she doesnt.  Ok so now for my question/concern.  She has a 6 yr old daughter from her first marriage [just went thru 2nd divorce], my husband has been around her and her daughter since he told me, and has even taken our daughter around both of them, without my knowledge and against my wishes.  They do things like they're a family, going to the park, going to the apple orchard.. those are only a cpl of things i know about.  It drives me nuts! They just went to the apple orchard today, the other day I asked him what if I said our daughter couldn't go?  He says 'I'd ask my mom to keep her and I'd still go, because if I can't do what I want with my daughter when it's my time with her, and this has been planned"  WTF is he doing?  He has been a good dad to our daughter, and he still is.. but for him to say he'd still go be with her and her daughter and leave mine behind because I say I don't want her around his "friend".  ???  I'm sorry this is so long, I just don't know what to do. I'm trying to just deal with it because whether i like it or not, or say i don't want it to happen.. he doesn't care.  Please let me know your take on this.. whether you agree or not.