is there anyway you could manage to get away for a few days on your own, to really assess your situation - sounds as though its all getting on top of you at the moment, and if you have financial worries on top of everything else, it just may feel like there's no light at the end of the tunnel. It may help you to have a day or two to yourself.
The other option is to talk to your husband about the financial situation - because he may surprise you and be more supportive than you think. On the other hand, if you do decide to leave, you won't be leaving him with any nasty surprises which he may use as an excuse to put you down to your step-daughter.
You've done a great thing taking on your step-daughter like that, and you clearly think the world of her - that being the case, i'm sure she feels the same about you - make sure she knows your not leaving her, and that you'll always be there for her - she's old enough to understand.
Hope things work out for you - but take your time before jumping to any rash decisions - and if you can talk to your husband about how you're feeling (and the finances), then you'll at least feel like a weight has been lifted from your shoulders. good luck x