I’ve found most success by figuring out how many calories a burn each day which leads to knowing how many calories I should be eating (this website is great for that), then I track what I eat so I know I’m eating the right amount and there’s no guessing. Some people don’t like that approach or don’t have to do it that way and can just eat “intuitively,” but I definitely am not of them.
If you think you’re under-eating you need to find a way to add extra, healthy calories into your day-to-day, not make it up all in one with high calorie to nutrient ratio foods. If you’re trying to lose weight you will be functioning at a calorie deficit, that’s how weight loss works, the trick is to go slow, steady and sustainable about it rather than crash up and down. Do the maths, even if just for a month or so to get a feel for it.
I’d like to add that thinking you’re undereating is natural, because you are. I wouldn’t be too concerned with the occasional binge day. I like the other comment about looking for foods that are more filling
Suggestion: Get an estimate of your total daily energy expenditure. Then, track the calories and/or macros in some foods/meals you regularly eat & do the math. If you’re eating at maintenance or a sustainable slight caloric deficit then you’ll know you aren’t actually under-eating. Or, if you are severely under-eating you’ll know how many calories/macros to add to your daily food consumption to avoid overcompensating later.
You might find this reassuring, and it could prevent you from getting into a binge/purge cycle, which it sounds like you might be flirting with. That is dangerous territory, my friend.
I find that knowing the rough estimate of how many calories I am consuming every day gives me comfort that I am not veering toward behavior that restricts calories in an unhealthy way. I also showed my partner the calculation of the daily calories I was consuming on 16:8 to reassure him I’m not under-eating. I’m eating at perfect maintenance and I don’t need to count calories to know this because I tend to eat similar foods on a regular basis. So I’m not suggesting that you count calories every day, just that you get a baseline of what you are consuming so that you know whether you need to eat more.