I need help dealing with a situation that involves "baby mama drama" things that go along with being a stepmom?

I was with my boyfriend for just about five years and we broke up in December 2009. We argued all the time and really never had the chance to grow up, so we peacefully went our separate ways and had no intentions of getting back together. We both dated over the next several months and ended up getting back together in May 2010. The girl he dated while we were broken up called two weeks after we got back together and told him she was pregnant. She slept with at least one other guy during her conception time frame that we knew of, so we talked it over and decided to see how things worked out. I feel like I have to point out that this girl is crazy and this is not a biased opinion. She slashed my tires, the tires on two of his vehicles, smashed all the windows out of one of those vehicles, and slashed his motorcycle tire and carved "payback" on it. This isn't even half of it. For the next nine months, we both got haring phone calls. She faked a miscarriage, "just to see how we'd react" and continued this insane behavior. After the baby was born, we had a paternity test and the chiild is my boyfriends. While it's hard to explain, I have no problem with the child. We see him every weekend and I adore him. Even though he's not my flesh and blood, it feels like he is. The problem is that it's so hard to wrap my head around all of it. I'm not ready to have a child, so it's not jealousy of not having one of my own....it's hard to pin down. My boyfriend & I have a great relationship and I know that if we've made it through this past year, we can make it through anything. It's just once in a while, the whole situation makes me so sad. I can't pinpoint why, but it's hard to cope with. I know one aspect is just being on guard for more "baby mama drama". She is now with one of the other possible baby daddys and he has helped to mellow her out where she's alright with me being around their son. I haven't overstepped any boundaries, I make sure that I clarify that the child is her's and my boyfriends and don't try to stake any claim. I just have seen her craziness before and I despise drama. Now I'm sure to those of you who are reading this may think I'm rambling or your first thought is to leave if it's hard, but that's not the kind of advice I'm looking for. I want some positive feedback. I love my boyfriend very much and he loves me just the same. We're making plans for the future (buying a house, getting married, having children of our own) and I'm excited for all those possibilites and that's why I want to figure out how to cope when I'm feeling down. I know the world isn't perfect, but I sit back and think that his first child should've been with me. All of his children should've been with me. I'll also point out that this girl claimed to be on the depo birth control shot, even had her mother in on this lie. He was still practiced safe , but there's always that chance and he was one of those small percentages. I'm not even sure if I've explained all of it in a way others can understand, but it's my attempt for some understanding and maybe a little encouragement. Thank you in advance.