I would start by designating certain nights as ‘eat at home’ nights, rather than trying to completely cut out the takeaways immediately. So say, Mondays and Fridays are still takeaways for now, but Tues, weds and Thurs you will cook at home. Then think of 3 very quick easy meals, even if they are freezer foods that you just bung in the oven, and make sure you know which one you are doing which night. So for example: Tuesday is chicken goujons, brown rice from microwave packets, and broccoli (with gravy if you like it); weds is veggie omelettes; and Thurs is frozen pizzas with garlic bread and salad. That way you get into the swing of cooking a bit without it being overwhelming. Other super easy things are beans on toast, jacket potatoes, stirfry from those kits in the supermarket, all day breakfast, sausage and mash etc.
Presumably you cook at the weekends already? Or if not, maybe use the weekend to try and cook something slightly more ambitious like a chilli or bolognese and then have it both nights so that you cook it on sat but do double the amount so you just reheat it on Sunday.
I do all the cooking at home so when I go away for work my husband really struggles with that aspect of home life, along with work and looking after the kids. I leave him easy to cook stuff for him and the children or they have batch cooked things from the freezer, or beans on toast. We very rarely have takeaways just because we don’t have the cash for that and also we live in the middle of nowhere so we’d have to drive 20 mins at least to pick one up and 20 mins back home again.
Be kind to yourself, you are still newly widowed and you have little children xxx