Lifestyle tips

I think heavy cream might be a 0-100 answer in this case.

Maybe start by lifting weights.

Trying to put on muscle will increase your appetite, and if you need to eat more to fuel your muscles, it will allow you to also eat enough more to also put on a little fat. This might help if the mental side is part of your issue.

If you have allergies try using an antihistamine to relieve them...I have also heard that it has appetite stimulating effects. Google periactin or something spelled similar to that which is a antihistamine ....it mentions people eating ravenous amounts and it even being used sometimes as a weight gain medication because of this effect.

Good Luck