Ditto everything Petal said. IME, what I found so dreadfully upsetting and insulting was that while DP readily acknowledged his ex was a vile spiteful bitch (and if anyone reading that wants to judge me - fine, go ahead - you don't know her and what she's done but I do) he would at the same time do all he could to appease her and keep her "on side". Anything so she didn't disrupt/contact any further .... yet even though it soon became evident that no matter how much he lay down and invited her to wipe her feet all over him, it didn't make a jot of difference and she still messed about, told dreadful whopping lies and played the stepkids as pawns. So really, what was the point in kow-towing to her ?
On the other hand, as his partner, who, in theory, he actually wanted to be with .... well, it was okay to upset me, cause me inconvenience and spend money we didn't have (meeting ex's demands) and which could have been spent in a 1001 different ways on our home and standard of living.
As a result I feel I have effectively been "firefighting" for more than a decade - anticipating how he'd react to ex and/or stepkid demands and manipulation and trying to see that off at the pass by establishing how we were going to approach a given situation before it happened so that hopefully ground rules - which were fair to everyone rather than those who shouted loudest - were already in place. Of course, living like that is bloody exhausting but I know if I didn't remain alert then there's a very good chance DP would give in to so much more unfair stuff just for the sake of a "quiet life" (so effing what if his ex sounds off at him?!?) and maintaining contact (not that giving in makes much if any difference). And obviously, I can't see into the future more's the pity and stuff happens all the time where there's not much I can do about it.
It's very very hard and I admit I carry a lot of resentment for all the consideration he's shown to the extremely unpleasant ex (and stepkids when they've been equally unreasonable). Bottom line, if you can't rely upon your partner to consider the effect on everyone concerned before he makes unilateral decisions and if you feel that you're almost an afterthought - because his 1st thought is how not to "rock the boat" - then it does huge damage to your relationship - and I honestly don't know if it ever recovers properly, even when stepkids become adults. We're more or less at that stage now and there's still sh*t going on in various ways .... contrary to what I used to naiively think about how refreshing it'd be once they "grew up" and the ex's influence became less strong (DP also used to wistfully imagine the same thing).
OP ..... it is good that you've had some acknowledgement from your DH but I'd remain cautious for now. I can't help wondering if he's sensed how absolutely peed off you are this time and it's his attempt at damage limitation. One way he could show you he was taking your feelings seriously would be if he sorted out this whole month of "extra" SD for a start - recognise that without notice and without agreement there's no way you should be expected to cover off the additional childcare needed. If he hasn't done anything about that, and is not planning to within the next couple of days I'd be very sceptical about his depth of regret. If he is serious about moving forward with more fairness and respect I wonder if he'd consider counselling with you ? ..... you'd soon find out whether he was genuine or if he was just paying lip service to your (understandable) complaints. It might also strike a chord if a third party was able to convey to him - or help him to see - how unfair his attitude is. I wish we could have had counselling because I think it would have helped immensely but we've never been able to afford it (due in no small part to the financial pressure caused by ex - and I'm not talking about child maintenance before anyone has a go but about stuff like her moving 2.5 hrs away and refusing to share driving, or her never sending clothes etc.) Oh the irony.