I am full of sympathy OP - you haven't had a fair break.
Lone parenting is tough - but the resentment of X must be very tiring too.
Keep in mind that you are doing the work but that in the end your X is missing out. The memories /experiences you have with children make them yours. Their connection with him will get smaller and less rewarding for him.
I agree with the advice above - that you need a routine of your own. Plans are your friends. Treat the summer holiday like another kind of work. (My mother always said that with 4 of us holidays were a change not a rest!)
Luckily, as a teacher you are a creative person - and have many skills that will be useful in making the summer memorable (in a good way).
Some we liked:
We found scrap-booking the holiday worked for us. All ages recorded our days with pictures/diary entries and cut n stick things in special books they selected themselves. They turned out to be great at cartoon drawing (thanks to their own sense of silliness and a helpful cartoon-drawing guide we got from the library one wet day.) Artistic awards might not have been won but we still laugh at the results and take great pleasure in looking over them.
Most important was the list they each made at the back of the book - a 'bucket' list of things to do before returning to school. (Picnic on the play-park roundabout - building the biggest lego tower in the world - cinema- sleep-out in the garden with friends - making own ice-cream etc.) We looked in this as we planned the day/week together.
Thumbs up to paper plates and lazy housekeeping days.
Most fun was in a wet week. We planned a 'hot' day for inside the house by creating suitably funny sunshine costumes and each prepared a 'foreign holiday' dish for dinner. Library and screens helped us put together recipes that were age suitable and not too expensive. Much laughing - but most of the meal was edible and the drinks with umbrellas were very colourful.
Map and compass walk - planned by eldest was good. We did need much kendal mint cake though...