I plan 3 meals and, as Tarragonsauce explained, link other meals. I plan my meals off the cuff in the supermarket depending on what looks nice, is good value etc. but I’m always mindful of what will have to be used straight away, what will hold for a few days. It helps that I can shop by myself. When I had the dc in tow I had to work it out on paper first.
Keep a good store cupboard of basics (pasta, rice, couscous, lentils, stock, herbs, spices, oils), honey, mustard, tinned tomato.
I use frozen veg in preference to fresh just because it extends my options. I keep bread in the freezer too.
But some veg has to be bought fresh so learning about correct storage is important. Bananas help to ripen other fruits and veg so need to be kept apart if your goal so as not to over ripen. A bruised apple can ruin other fruits.
Buy lettuce with the root intact and it will last better. It’s very easy to grow if you’re inclined.
It does sound as if your fridge might not be cold enough. You can buy a fridge thermometer quite cheaply to check.
Someone asked about storing potatoes: somewhere cold, dark and dry like a shed perhaps. Keep them up off the ground and Cover them over with an old blanket. In olden days people dug pits and insulated them with straw.
Omelette, stirfry, risotto, soup and stew are all good ways to use up odds and ends.