I have some similar problems. It isn't easy at all for me to keep weight on, and it's easy to lose it.

Here are some ideas:

If you do some research on the digestibility of various foods, it might give some indications. There also seem to be individual differences here.

Some specialist MDs deal with this kind of thing. They know which foods are best for patients with malabsorption syndromes, surgically reduced digestive systems, etc. They can probably give some good information. [I would be interested in more of this, if you find anything.]

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I have found it extremely helpful to have a reliable and thorough reference book, one that covers a lot of different foods, and has good calorie charts.

I learned a lot this way, and dispelled a number of myths and preconceptions that are popular.

Healthful Mediterranean diets often include a higher intake of vegetable oils than most people imagine (mainly olive oil, nuts and seeds).

All oils have about 120 calories per tablespoon.

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Flavor and enjoyment help a lot. If you have oats, for example, and they are not prepared and seasoned that well, they can be pretty bland or so-so to eat. On the other hand, if you do them right they can be *really, really* good.

It helps alot to learn to prepare them in the most attractive, optimal ways. It isn't that much harder, longer, or more expensive to do this. You just have to do a little research and learning.

The same applies to other foods.

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It sounds like you are often in a hurry.

You may or may not believe this, but taking even a minute or so to relax and get into a peaceful state of mind, and to eat in that state of mind, really does help digestion.

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Being in an emotionally upset or angry state of mind hinders digestion. It helps to have a calm or even emotional life, and a positive attitude, and good will. (Again, some people would undoubtedly not believe this, but I have found it to be quite true, and I have heard some pretty insightful people mention it as well.)

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Potatoes are less fattening than most people think. One can check a calorie chart, and compare with oats and other foods, and find out the facts pretty easily.

The reason most people get a lot of calories with their potatoes is that they usually put butter, sour cream, or other calorie-intense toppings on them.

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If you wolf down food (thick rolled oats would probably be included here, among many other foods), without chewing, the particle sizes in your stomach end up being larger; and this often means a lower level of utilization.

Chewing well also adds some digestive enzymes (though this is probably a smaller factor).

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If you drink some calorie-rich liquids during the day, they can add a lot of calories.

Nuts and nut butters are healthful and rich in calories.

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I find that eating a big meal at night means not getting such a good night's sleep. I sleep much better when I eat fairly lightly, and well before bedtime. Others have noticed the same thing.

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There are some 'grain milks' that can be flavored, and made to taste quite good. They are very digestible. Oats and wheats can be used, among other grains. Google can help with more details, if they are of interest.

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Good luck with it all. Hope some of this is useful.