It took me ages to get good at using up leftovers but i pride myself on it now. Here are some of the things i do.
I have a load of clip top boxes of various sizes and bits left overs go into those on the top shelf where they can be seen, or if its a full portion of something it goes in the freezer. If it's something dd likes she gets it for her lunch.
On the nights when i eat with dd and DH eats later, he will get the single portion of whatever is in the freezer.
If you have uncooked potatoes going over, make a big pot of mash and freeze in portions.
When i make apple/fruit crumble i make double or triple the topping. It keeps for ages in the fridge and if fruit coming to it's use by stage, i can make either mini crumble in ramekins or a big one depending on how much fruit to use.
I've never managed total meal planning, but a bit here and there works for me.
When i decide on dinner, i first look to what is to be used up, and then work around that. Ditto for lunch.
I always make double and freeze things like spag bol and curry. And i made a point of working out how much rice or spagetti we actually need so that there is no leftovers.
I always buy a large cauliflower (same price as the small ones) and make double chicken korma with cauli, and freeze half.
I only ever serve cabbage with corned beef, and we only need half a cabbage, so i make sure i have carrots and buttermilk and make cole slaw for DH's sandwiches. Because i have buttermilk in the house i also make 2 brown soda breads and freeze one.
I shop at the butchers once a week, and freeze it all, i normally have several choices of meat to pick from (i.e what ever will go with the leftovers.
You will get better with practice.