A trick I used was to eat only a certain volume. I had this very reasonably-sized bowl and I'd fill it to the brim (exactly) with food and that would be my lunch for the day. I also always had the same breakfast: a mug of chocolate milk and an apple.

I cut out all snacks and such, and just tried to eat reasonably healthy foods like rice and tuna, and mashed potatoes with veggies, just random stuff that wasn't chips and gummies.

Yeah, it's hard, but you can definitely do it! If you need help and don't have anybody close willing and able to guide you, maybe you could look at getting a nutritionist? Not sure how available that is...