Start easy. Use the money saved on takeaways to get some kitchen tools. Stick blender. Electric blender/slicer/grater, can’t remember the name Russell Hobbs make them - makes adding veggies so much easier as the prep is fast and you can cut them to tiny sizes if there are any the kids often refuse. Sandwich maker. Some glass storage boxes with lids to keep leftovers in the fridge. Where possible get things that can go in the top rack of the dishwasher.
Get the kids to help make a huge spaghetti bolognese with lots of veg and red lentils at the weekend, have it that day, leave enough for another meal and freeze portions.
20 minute veg soup - slice up potatoes, onion, boil in stock and add in frozen broccoli florets. Blend with cream. Leftovers will last a few days.
cut veggies, fry in olive oil, add seasoning, a teaspoon of sugar, tomato paste and passata. Make enough for two days. Eat one day over pasta. Next day mix with pasta, cover with cheese and bake. Add a can of chickpeas if you don’t mind them.
toasted sandwiches and salad.
tomato soup - boil onion, tomatoes, stock and add a bit of cream if you want. Add some herbs if you want - get the kids to help choose snd plant them. Blend. Serve with cheese toasties.
keep a few pizzas in the freezer - will save a lot over ordering them and you can see the ingredients.
Look into tray bakes - they don’t need much attention while cooking.
Best wishes OP. It must be super tough. Do what you can and don’t beat yourself up.