I'm going through the exact same thing, except I've pretty much figured out for myself what does it. The more difficult to digest the more energy drain I feel. Unfortunately this has placed fats at the top of my list of avoidance foods good & bad since they all drain my energy the same, it's all that much worse when I'm actually sensitive to the oil source.
Sugars & carbs initially don't do much but if I eat too much at once especially without thoroughly chewing then I notice about 30 - 60 minutes later I get a massive crash, which to me feels almost like a blood Sugar spike & drop. Hands & feet sometimes go cold as well.
I'm doing best with protein & vegetables, the only limitation with meat of course being the fat content but I find I'm still better with it in moderation than taking straight oils.
Just my experience but maybe its of some help to you.