I bet it feels a lot better just getting it all out! The frustration is coming through loud and clear on your posts and you sounds utterly at the end of your rope. 
When you have calmed down a bit and got your head sorted, you need to talk to your partner. Step parenting is hard, I think its even harder when your a step-mum and your partner is the RP (that's the category I fall into, and even though DC don't see their actual "mother" very often, her choice she is very busy
I still feel like I'm sidelined when they do actually see her, once a year) if you have a child that has just been sent to you "to be sorted" it must be very hard for all of you.
I'd strongly suggest counselling, it's so helpful if nothing more than a place to just let it all out in a safe place for you. I know in the beginning I had some negative thoughts and I felt so ashamed for some of what I felt, worried DP wouldn't understand (he did, I'm very lucky) understanding through a therapist where these thoughts were coming from (fear based) has certainly contributed to us being a strong family and a strong couple.
But, my DP has always been very much on my side. He was always of the view that we are adults and children never dictate what happens in our home, and that no ones wants (NOT needs) would ever come before anyone else's needs.
That's what needs to happen IMO in a blended family, and you have got five years of learned behaviour to change but IF he supports you and you work together then there is no reason why this can't turn around. He does have to help you, and if he doesn't then I'd question why your continuing to stay and I really don't say that lightly.
Stepmonster is a good book to read, if you haven't already may give you some insight into why your feeling the way you do.