My boyfriend has a baby with ex bully?

Me and my boyfriend have been togther for nearly four months now. I'm 19 he's 23. As soon as we met we hit it off. The first night we met he mentioned he had a baby son *Matt. I never thought too much on it. I found it a little weird, but pushed it to the back of my mind. Third time we met up something clicked, i knew a girl i'd despised at scholl had just had a baby with the same name, i thought it had to be a co-incidence as she was a bully to most people throughout school an awfull girl inside and out! However it turned out it was his child. He'd slept with her a couple of times, that was all. I was horrified but already even though we'd not known each other long liked him so much. We carried on together but the longer were together the harder it is for me to handle. I got freaked out anyway as its hard for me to take that he has a child it freaks me out so bad. and to top it off its with her. The longer were together the more i think about it. I'm worried as the child gets older i'll come to hate it and i dont want that. This girl is awfull and the babys a reminder that they were once...y'know. andd im jealous and i cant stand it. a lot of the time im ok other days i dont stop thinking about it. What do i do? I love him to much ..