When I was younger they wanted to put me on steroids because I was so small, luckily they didn't.
You actually make a good point with having several small meals. Everywhere I go I get a kids meal and I can never even finish, lol. Since I can't eat a lot at once it's probably a good idea to eat a little... a lot.
I might not be the best help here since I have had the opposite problem my whole life of overeating/binging and never really feeling full. But I agree with the small snacks/meals thing. When people have been starved for a long period of time doctors introduce food to them slowly and in small portions. Because like Frax said, your stomach can't handle large amounts when it's not used to it.
Maybe you could try finding some calorie dense foods you enjoy and try eating those throughout the day. It takes 3,500 calories to gain a pound. You can calculate how many calories it takes to maintain your current weight here (https://www.mayoclinic.org/healthy-lifestyle/weight-loss/in-depth/calorie-calculator/itt-20402304), and that can give you a guide to around how much more you should be eating to consistently gain weight. Though of course you won't be able to up your caloric intake too much starting out. Since it takes time to adjust to eating more/less. But maybe since you said you forget to eat a lot, you could start a food diary to keep track of what you eat. And keeping up with the diary will help you remember to eat.
Sorry if all the advice is unwanted. I just know the struggle of weight issues and wish I had known some of this info earlier in life.
Good luck with trying to better your diet!