I'm afraid that yes you are (expecting too much). And perhaps your expectations of him wanting to stay involved don't match the realities of what this means to him. Have you asked him what he means when he says that and listened to what he says?
Basically as a single parent you simply have to get on with it. When it's your time you have to be responsible 100% for your children and you can't hope or rely on others or expect them to help you out (even if they are the father of the children concerned). And this is regardless of how little sleep you've had, how ill you are, or whether you are physically and emotionally capable to do so. Because there is only you.
You are no longer a couple who shares your life and helps each other or makes sacrifices for the other. You are now two seperate and single parents. When it is his time he has responsibilty for the dc, when it is your time they are your responsibilty.
I think that is reflected in the fact you have only asked him to babysit after your other arrangements have fallen through and then expect him to jump at the last minute and cancel his own plans.
Maybe if you asked him well in advance. And maybe if you made him feel more like a carer and parent of your dc rather than just an emergency babysitter of last resort...
My sister once rang me at 7.30 at night saying could I come over and babysit as she had a works do and her husband had been delayed at work. So she basically expected me to bundle my tired ds in the car, drive half an hour to her place incuring petrol costs, babysit for free so she could go out, and then drive home with ds at some unknown point in the future depending on whethe rhe dh made it home before she did.
I was absolutely fuming at her expecting me to drop everything with absolutely no warning to help just because I was doing 'nothing' in her eyes. She didn't actually even ask if I was busy, clearly too flustered and wrapped up in her own requirements to consider what I may be doing.
Plus I know that if I ever asked that of her she'd tell me how tired she was and how she couldn't possibly bring her children to babysit my ds. In fact if she has ever babysat I have had to ferry my ds to her and back.
So her evening out was clearly more important than any thought for me or my ds. And that fact she would never ever do that for me just was too much I'm afraid.
Fortunately I was down the garden bringing in washing so didn't hear the phone ring when she called. But I was even more shocked to find out that she had rung my parents to say could they walk round and tell me she needed a babysitter and could I leave now. And because I wasn't answering the phone my mum offered to go babysit instead.
So on some level I can understand why your ex may not have been jumping to your request when you called, especially is he had other plans already.
And my ds is 11 now. I don't have an ex to give me a break ever as ds's father decided he wasn't ready to be a dad yet before he was born and has had no involvement at all.
The only time I ever get a break is the rare weekend when ds was at cub camp for a couple of days, or when he's been on a school trip (once). Oh and the two times he has been away in the school holidays when I have worked till gone 8pm each night in order to reduce my childcare costs and have more time with him when he is back. So hardly a break then either. Very occassionally he may go overnight to a friend. And I can't afford a babysitter (or a night out I might add).
And my own family, pah. No help there either. I find it so much easier now I have stopped waiting, asking, hoping, they will offer some/any kind of help.
You will probably find the same relief too if you stop hoping and expecting him to help you out and simply accept this is how it is.
I'm not saying it's fair, or right, or easy. But it is the reality of being a single parent I'm afraid.
The sooner you get overnights with your ex sorted out the sooner you'll get some well earned rest and a break. Oh and perhaps go to him as your first port of call with plenty of notice to 'babysit' rather than last minute may help. Treat him as a co-parent, not a babysitter. If he says no then you have time to find an alternative, and if he says yes then that's a binus for the dc and for you.
Hope you're feeling better now and got some sleep and the dc are better too. It really isn't easy but becomes easier when you stop relying on other people and just rely on yourself. It's taken me a long time to get to this point though, and I'm so much happier for it. Hope you work things out for the dc soon.
Best wishes