Under eating

The thing to watch out for is whether or not filling up quickly is physically or psychological in nature.

If it's a physical thing, and your stomach just doesn't hold a lot of food, then calorie dense foods like the list above (nuts, nut butter, oils, butter, heavy cream, avocado, etc) would likely help.

If you aren't eating enough because the foods are bland or less appetizing to you (which is kind of what seems to be happening since you ate more by adding peanut butter to your breakfast) then just making the foods taste different through higher fats might help. If you ask me to eat a whole chicken breast, it won't happen because I don't like chicken much. If you ask me to eat a plate of chicken tacos, it'd done before you even have to ask because the preparation is more appetizing to me. There are some foods I just don't like enough to eat the whole portion, even if I'm starving when I start eating.

If it's psychological, as in you're feeling full because you feel guilty eating enough, then that's something to watch out for and could get worse as time goes on. I would say that most people, especially just starting out, feel guilty sometimes when they eat. It's normal and probably not an issue if it's just a bit or just sometimes. But if you feel guilt to the point that it's sort of taking over your life then it's too much.

There are just a lot of mixed messages in this post and you may not even know exactly what's happening yourself. That's okay. It's good to sort through some of this stuff and see what's happening because it will help set you up for success if you understand the hows and whys of your eating patterns. But do start keeping an eye on why you are eating so low and it might help to make things clearer for you.