Am I just a rebound?

Its a long story. I have been with my boyfriend for 3 years now, i am currently 25 weeks pregnant with our son. My boyfriend has a 4 year old daughter with someone else. From the beginning i started dating my bf in march 2004 we were together for 2 months before his ex and the mother of his daughter found out that he was with someone so she wanted him back and he went like an idiot. I got over it, and moved on. he called me a few times a week the whole time they were back together. they ended up splitting up in 2 months and he started calling me all the time then. i didnt want to go back to him right away and didnt until 3 months later after they split. Things were ok between us but the only thing was i could only see him on the weekends because i was working and in school and the only free time i had was the weekends. 3 months into our second time around the kids mother shows up at the house. when earlier that morning my boyfriend told me he was going to work for his uncle and boom who appears the ex. she started her shit saying they were still sleeping together and trying to work things out between them. I didnt believe her at first but it took him 8 hours after that scence to get in contact with me and it was by email i called him when i got the email and he never answered his cell phone. he called me the next afternoon to apologize for the baby mama drama. There was a few times we were having sex and she called on the phone and he answered it. She came to his house and they went trick or treating with there daughter when he knew i just got back from cali for 2 weeks and wanted to see him and he lied to me and said he was going to his sister halloween party. the daughter gets special help with speech so a therophist comes to his house once a week to have sessions with the daughter, i understand the mother should be there it is her daughter, but why did he always have to hide my belongings in his when she was around. he never told me that she was actually at his house i knew because she smoked and she was the only one who smoked and i seen the butts in an ash tray. He always went into the other room to talk to her on the phone. he never asked me to go bring his daughter or pick her up from the mother. She told me infront of him that he cheated on me 3 times with her and he even ate her out. He still till this day say she is a liar. now i am starting to think she was telling the truth. Help me please