You might not be overeating at all. Tuna and vegetables have a lot of volume but less energy than pasta, soda, etc. Folks who are naturally lean tend to eat more frequently and more carbohydrate so I would be unafraid of potatoes and possibly rice and peas.
If the appetite is intrusive I would plan proper meal courses, e.g.:
soup/salad -> protein/starch -> fruit/fat
Looking forward to that is better than drifting from one snack to the next IMO.