I'd have a hard time being underweight without willpower. You just have to give in and eat.
I used to be underweight after being overweight and losing weight but it was because I had an eating disorder and felt unworthy of eating (just another side effect of inceldom depression, back then I didn't have a word to describe my condition but it always comes down to being subhuman and developping issues with self-worth because of lack of positive reinforcement from a young age).
If you want some regular advice : loosen up with the diet, allow some junk food if you want, if you don't have much money, bread, jam, spreads like peanut butter, butter and margarine, pour some oil, eat more carbohydrates (refined) and fat and you'll gain weight back for cheap.
If you struggle with appetite or feeling full, eat regularly small amounts and use discipline.
If you don't see the point in eating because you want to die, then I just send you my non-religious prayers.
Junk food and food is general is a damn good cope for a lot of us here, so it's usually the other way around and people have a hard time being at a normal weight (although I'm a normal weight, if not low-tier right now since I no longer see the point in trying to be in shape, I'm eating whatever candies and stuff I want now so the weight will creep up eventually).
