I dealt with this when I was a Health Visitor. It was sad to hear such stories, but in the end, not only you, but your DGCs will suffer.
Your age is irrelevant - how you feel is what's important. If you don't tackle this now, it will build, and something will happen that may cause all of you grief.
So give them notice - maybe in writing - maybe by saying you need to meet them quietly for an hour or two to talk things over. In advance, think carefully about what you can offer. Work out how many hours, how many kids, what days etc. Make it clear that you will step in for an emergency.
If you can cope with the status quo until the New Year, and talk to them in the next week or so, that is plenty of notice.
It is quite reasonable to split childcare between nursery or other providers and you.
You can explain that you are not blaming them for not noticing, as you have always tried your best, but now it has to be done differently. Of course they will be upset for a bit - you don't say how tight money is - but carrying on like this serves no-one in the long run.