What doesn't go to waste goes to waist

I am frugal. Not to the point of being stingy, but I try to look for good deals. If I go shopping I'll try on clothes and then leave the store without buying anything, then if I'm still thinking about it throughout the day I'll go back and buy it. I'm often thinking of finances. This has transferred to the plate.

When I am eating I can't leave anything on my plate. This isn't because my parents always forced me to eat everything on my plate. On the contrary, actually, my mom always said "what doesn't go to waste goes to waist." It's because when I'm eating I think of how much money I paid for my food (especially if I'm eating out). I don't want to waste the money I've spent so instead I overeat.

I talked about this problem with my dad and he had some great advice. He said the value of food isn't in how much it cost, but rather the nutrition it gives. Can I change my thought process to give up thinking about how much money I've spent? I will try. And I can start not by leaving food on my plate (that may take me a while to get to that point) but rather by taking smaller portions. Eat what satisfies me and let that be enough.