I agree with SM. There are 3 things you need to deal with here, all of which are very difficult, and unfortunately are intersecting.
1 is your own fertility situation. It must be incredibly frustrating but 6 months really is not that long in the grand scheme of things (unless you're 40+). It seems to be taking a very central place in your life at the moment and understandably you're envious of other women when they manage to get pregnant and you feel like you can't.
2 is the fact that your step-daughter is pregnant, which is going to put a strain on your family, your DH is going to have to deal with becoming a grandfather whilst actively trying to become a father again. There's also the fact that she's 16 and living under your roof, so maybe you're holding yourself a little bit responsible for this because you feel like it's reflecting on your parenting?
3 is her risky behaviour and her attitude towards this pregnancy, complicated by 1 because you feel she doesn't deserve it and you do.
As for what to do you need to think very hard about all these 3 things and how you're going to tackle them. 1 is your issue and no-one else's. 2 you need to support your DH and his daughter and work out what the best way of tackling the whole situation is as a co-parent. It's okay to express disappointment etc but make sure that's related to the impact on her and her life and NOT the ease with which she's got pregnant. She may be putting on a brave face by saying she's going to get herself a council house and live off benefits. 3 for the sake of her baby you need to support her in having as healthy a pregnancy as possible so remove yourself from the equation and support her as much as you can in doing that.
Finally you seem to have a very negative attitude towards your DP's ex. I think perhaps that's something which needs addressing as well, either by you alone or in conjunction with your DH if there's a reason behind that.
It's okay to be upset and disappointed. It must be a million times more difficult that it's your step-daughter, living in the same house and it's accidental but for everyone's sake you need to separate out your own feelings from this at least outwardly.