I've namechanged due to what I'm about to say and otherwise having a public profile.
I could have written your post almost word for word other than the prep school considerations - but share the same frustrations and fury you do about "unbearble" stepchildren (in my case) and yes, their vile bitch of a mother (agreed - that sounds bitter but twisted vile bitches DO exist in the world, and why should I apologise for the totally unnecessary malignant effect upon my life and that of my children & partner she has ?).
The logical part of my brain knows - as Carnmere has said - that in order for my relationship to flourish I need to find a way forward with my skids (in my own mind I mean - practically, day to day, there is no conflict between me and them. I am very good at gritting my teeth and acting, albeit that this comes at the cost of unbearable resentment) - and I also know, and think it terrible, that the totally unattractive and often damaging (to the rest of us) behaviour they exhibit is almost certainly due to their mother's influence. For that, I feel sorry for them. But on the other hand, I am not prepared to sacrifice the rights & needs of my children in order to "keep the peace" (though I usually do). I find that grossly unfair and I hate it.
I would NEVER ask my partner to choose between us. But for years I have struggled internally about whether or not I can continue to be in such an unhappy place. Before anyone else says it - I did of course know he had kids, but what I didn't know (and would have been thought pessimistic and/or a shit stirrer had I suggested it) was how these children would develop in years to come - hugely influenced by systematic alienation from their "mother" (quote marks because she isn't what I'd call a mother and does NOT have the children's best interests at heart .... is only interested in scoring points against someone she wanted to split up with many years ago) .....
..... it is all such an awful conundrum. I exist through it all, year after year, hoping for some sort of miracle regarding the ex and the skids. That looks increasingly unlikely as the problems we face worsen year upon year (and they are many, and significant - too much to go into detail about, but almost unbearably stressful for all concerned) and I am worn out from not only being my partner's support but from also having to deal with my own feelings on the matter, which my partner often overlooks (or considers "minor" compared to his).
The only major bones of contention within our relationship are caused by his "baggage" for want of a better word. I wouldn't even say contention so much, as simple stress. But it wears you down and tires you out, with no end in sight. I hate, hate, hate it all with a vengeance and there are occasions when I admit I "hate" my partner for having dragged me into this (I feel he often hasn't handled "the situation" very well, for fear of rocking the boat, or for fear of contact being further disrupted. Irrespective of how this affects the rest of us).
I dream of getting out - when I would miss my partner terribly - but I also know I would feel like a huge weight had been lifted from my shoulders. And there lies the rub ...... I am in my early 40s, and no way, any way, even working full time could I ever afford to buy even the tiniest flat for myself and my children now. (I was a single mother before for several years and could only just manage then ..... now, any sort of property anywhere is completely beyond my reach - even a studio flat which of course would be completely impossible for 3 people). Ditto renting. I have no objection in principle to a council home but the fear of the unkown in that regard stops me making the move. That all sounds terrible, but I just can't put my kids through that ...... or at least I can't at the moment. Maybe I'll get the incentive to go when I really am on the verge of that nervous breakdown I feel is waiting for me around the corner ?
Alternatively, I daydream of winning the lottery so I can get out instead and still maintain a normal (not fancy) standard of living for me and my children.
There does seem to be NO answer to our situations. Believe me I have spent years (probably about 4 now) seriously thinking of how I can resolve the situation one way or another. Due to their circumstances and the way in which they are being brought (dragged?) up the skids won't change any time soon (hopefully they won't be completely selfish, spiteful, lying adults bit with their mother as role model I don't hold out much hope) and I refuse to "change how I view their behaviour and attitudes" or some such similar psycho-babble, because I think to "accept" that would be morally wrong, hypocrotical and very unfair on my children who are being brought up to consider other people, to be honest and so on.
Can I live with this ?? ....... no, I can't. I feel that one day there will be an "explosion" of some sort, as you have alluded to. But neither can I leave right now. It's not a question of poncing off someone else, it's a question of all parts of my particular, personal equation simply NOT adding up to being viable outside the practical framework of my relationship. I am not trying to "diddle" my partner, nor decieve him. He's unhappy too, because of the "baggage" situation and there's very little that I or anyone else can do to change that.
I exist, I don't live. Like you, I snap and snarl when I shouldn't do and then feel terribly guilty.
Carnmere's right .... we probably should both be going to Relate or counselling of some sort. Of course that has occurred to me over the years ...... but that's a can of worms in itself. It's physically impossible for me to go on my own for example, which might help me get my head straightened out before speaking to my partner - and TBH, he's not stupid, and he's said enough (and we've talked enough) over time acknowledging that the situation we live in (caused by someone else's obsessive spite and bitterness) is appalling. I just don't see how counselling would help when the things I'm so unhappy about (ex's & skids' extreme behaviour and its effect upon us - emotionally, practically and financially) are far beyond what most people would consider as "acceptable" and therefore, I could never "come to terms" with it.
I suppose I have this very vague notion of a) winning the Lottery so I can get out or b)managing to exist in this horrid limbo-land for the next 8 or so years until the skids are adults (thought at what cost to my sanity or relationship I don't know).
Bottling ....... I'm sorry, I seem to have taken over your thread with my own complaints and I don't know if I have said anything helpful to you at all. Maybe you might take a very small crumb of comfort from not being alone ?