nomedoit: I do know how I feel, but I am trying to work out why I am feeling that way. Is it pure jealousy? I would love to go to Italy too, and even more, I would love a (small) child free weekend to go and explore. Is it just the rejection part? I am, after all, "only" dsd's stepmother. Of course her parents mean more to her. Is it irritation that dh agreed (well, he was ambushed by the suggestion, rather) without talking it through and nailing down the bits that no-one is clear on? There's a lot going on, really. And I was trying to sound out whether any of the aforementioned reasons would be unreasonable (ha! not going near AIBU with this one!)
It is not yet booked (which is why we are not certain that ex's partner is going. but going on past experience he will be. things like this are just not mentioned. ex in the past has sworn the children to secrecy over things - never to be mentioned to dad - and her relationships are one of those things. so it wouldn't occur to dsd to mention if he was going - it's all natural, as it is her mother's partner.), but it is Italy that dsd has set her heart on.
And that's another bit I am trying to work out how I'm feeling over. we asked what treat she would like for her birthday. this, traditionally, has been dh + relevant child - out for the day, child's choice of treat, extras out etc, then back for their choice of dinner and cake made by me (and dsd has put me through my paces with cakes, i can tell you - she still loves elaborate decorated cakes, and she has free choice each year over which design I massacre lovingly craft)
so, for the answer to come back as "oh, well mum and I have been talking and I think a weekend in Italy is what I want" was 
at the least. and dh was taken aback too.
now, technically speaking we can afford it, but I am still at the ask. especially as it is not part of her present, but an extra.
mj: to a degree, I agree. I do not care what dh's barking mad family think about me. nor his ex. but i do care what dss and dsd think about me. that, I think is only natural, as I care about them. and the things that are said and implied are of the hair-curling variety (I am sure you are well acquainted, sadly). dsd in particular is, due to her disabilities, very impressionable. and it does sadden me that she is getting very conflicted, untrue messages. but yes, I probably need to disengage, for my own sanity. I have started doing so, but I think dh feels that that means I care less. I do not, but I cannot carry on giving my all to and for people (they are, of course, no longer children) who do not give even the slightest bit back.
foolio: hmm, spa you say? sounds lovely, but I will be at home with small children, while dh yet again swans off child free. I understand what you say, but tbh, dsd has not really ever been let away with this type of thing before. yes, she has always come out with "I wish you and mum were still together", and she is very good at playing one off against the other.
but there have been times when dh and ex have worked together on this (sadly not enough times) - usually when dsd is trying to use us to get one over on her mum, and dh backs up mum instead.
so I really can't work out where this has come from. whether it signals a shift in our relationship, and this is a forerunner of things to come, i don't know. but I do agree there is an element of teaching dsd about other people's feeligns that is being ignored here. this just isn't somehting that should be encouraged, if you ask me.
if she has a reason for not including me (and I can't think what that would be) - we should be talking about it, not sweeping it under the carpet and not mentioning it. Not Talking about it surely gives the whole situation more power than it should hold?
I agree with mj too - I don't think dh is being an arse for the sake of it. he is in a very difficult position. he has been invited along (because tha tis how it is, no matter who is actually paying) and for once will get to be with his eldest child, on an importannt birthday. I can see why he has agreed to it (and of course, the whole suggestion came out of left field, had already been discussed between dsd and ex, and presented as the only option. he was ambushed into it, and had not much choice int he matter, unless he wanted to be cast in the baddie role again - and you only get one 21st.
I am close to feeling as though this is the last straw, tbh. I do not want to be ruled like this for the rest of my life. I understand what you are saying re: stepping back, mj, but I don't think that would work for dh & I. he would feel as thoguh I was rejecting his children, i think.
jesus, it's all so fucked up.