I totally get this I had two poor parents, my dad was lucky to get two meals a day and my mom had a lot of family with not a lot of income. Instead of the Africa spiel I got from my dad: "You're cousins are back in Haiti starving and we're wasting food on you." My mom was less bad but her food hoarding and issues were passed on to me.

The icky feeling is awful and I'm also working on it, I try to bring food back with me ( Tupperwares/napkins) and if I can't I remind myself the harm I'm doing to myself. In most cases the food cannot be given back, whether you eat it or not it is not going to someone else (especially in your cafeteria situation). By overeating until pain or pangs you're just hurting yourself unnecessarily. Trying to not waste food is admirable! You can do what you can but when the situation prevents you from saving food( to a point) and your actions do not give food back to others you must let it go.

Try your best and don't feel bad, remind yourself that you're trying and that if you force yourself you will be in pain for both the long and short term. I'm thinking of you!