This is not meant to be a flame, but let me say what I am thinking.....
We all know you have a lot of trouble gaining and maintaining weight. Much of this is due to some medical conditions - treated and untreated.
Due to your genetics and possibly your medical conditions, your metabolism is very fast. Which then allows to you eat foods most people cannot - without fat gain.
This seems great.
But, in reality, I think you might have better luck if you try cleaning up your diet. Fast food, we all know is not healthy. Just because you *can" eat with - without fat gain repercussions, does not mean you should. This goes along with sweets - ice cream, candy, etc.
I know most so-called "bad foods" are calorically dense - which is good if you are trying to gain weight. But they are also very nutritionally void. Your body CANNOT use them effectively. So you may be getting a good amt of calories, but they are not GOOD calories. Calories your body can USE to make it bigger, healthier, stronger.
I seriously think you should look into CLEAN BULKING foods.
Just my 2 cents, for what it is worth.