I'm afraid for me it's largely genetic and I think this is true of a lot of people. I'm sorry, OP! I am, and have always been bordering underweight. Most people would say I eat what I like but I'm not sure how true it is as you never really know what others are eating. e.g. if there are chocolates at work I'll just eat them but the rest of the day is probably OK. This is a typical day but am not suggesting it's particularly healthy...more to give an idea.
Breakfast (normal time) - two wheatabix, smallish amount of whole milk, handful of berries and dried fruit on top. 3 wheatbix would be too much.
Lunch - sandwich with cheese and real butter, crisps, banana. But I'm always hungry so I eat it throughout the morning (again this is why people think I eat a lot but really I just eat it early).
Dinner is early with DS (I finish work early). Nearly always home cooked but simple. Always carb based. If I have pasta it's at least double the recommended amount. But a portion of sausages for me is only 2. Plenty of veg. This is probably a poor example as people I know tend to find cutting out carbs helps. But I am uncomfortably hungry without them.
I go to bed early but if I'm hungry before bed it's either toast or an unhealthy snack.
I never skip meals (I couldnt) but also never eat to uncomfortably full. I'd have 2 rather than 3 courses for a meal out as a preference.
I don't drink alcohol or soft drinks but that's just preference. I drink lots of water. I'm fit enough to run 5k but don't do it often. No other exercise but I have a lot of nervous energy so walk a lot. Takeaway every couple of weeks. Would never restrict this but natural preference is probably on the healthier side e.g. margherrita (can't spell) not meat feast.
I think my body sort of knows what quantity to eat. e.g. when I was pregnant I was extra hungry at breakfast (am thinking a couple of 100 calories more). The rest of the day I ate the same as normal. I was just me with a bump and just went back to normal afterwards.