boyfriends son

I've been in stepsons life for nearly 2 years, but I'm finding it increasingly hard to be around stepson. I know I'm a horrid person for feeling like this, I feel so ashamed, but things with stepsons mum have always been difficult. And now I'm just really jealous I guess. That she was able to have a child within a month with my boyfriend, and I haven't! Boyfriend mentions this A LOT, which I have brought up and told him it makes me feel like rubbish!! We've took things slow, I was introduced to stepson after 6 months of being in a relationship, we've moved in together etc yet he jumps into bed with her and she's pregnant like that. It just seems so unfair and I'm fed up. I can't talk to anyone else about this as my friends don't want children right now so don't understand the hurt I feel when I get a negative constantly , or my period.