I don't get how I'm supposed to eat so much

Bit of a weird vent but whatever.

According to my pediatrician 2 years ago, and before I turned 18, I was 'dangerously underweight' so I ate more, to no avail. So started counting my calories a year ago, and so far, I haven't eaten more than 1000 calories a day. I don't get how I'm supposed to eat around 2500 calories a day according to my doctor, that's just insane

3 ENTIRE meals a day? How? I eat one then I'm fine. Bowl of cereal in the morning? Set for the day. Dinner? Set till next day's dinner. And I'm making sure they're recommended meals for people in my situation, not unhealthy shit or empty calories, etc.

I eat more than one meal a day then I'm sick to my stomach. Usually I eat half a meal a day, which is around maybe 200 - 400 calories, because I never finish, because I get too full or sick. If I eat a whole meal, it's usually 600 or so, and if I'm really pushing it I can go higher, but usually not above 1000 calories on my tracker

My weight is still bad, a little better, I used to be able to count my ribs but now I've put on a few pounds. I feel fine physically, although my blood pressure is always really low, probably because of this. I don't think I have an eating disorder, but whatever the fuck it is is weird