Why can't I gain weight?

I'm a 19 year old male, 5' 9", 119 pounds on a good day, and miserable. Evidently they've checked and rechecked my bloodwork and diagnosed me with hashimoto's thyroiditis which would mean my thyroid is underactive if anything. I've been on the medicine which i ume returns my metabolism back to normal. However I have a terrible habit of ignoring basic physical laws. Consuming upwards of 2000 calories a day for someone of my weight, I may put on 2-3 pounds at the most. However, given one or two days of regular eating which would be three to four meals, I suddenly lose it all. I've spent (no exaggeration) thousands of dollars on fast food in my quest to frantically stuff myself full of calories only to return to where i started around 115 pounds. I overeat to the point of wanting to throw up sometimes, all to gain that one pound, let alone get to a decent weight that hides my skeletal m. I mentally cannot grasp the concept that in order to maintain even a measly 122 pounds, I need to continue to eat until I choke for every meal. Every bit of advice I have gotten has simply been to eat more calories or drink weight gain shakes. What they don't seem to understand is that it overall costs so much money to simply put on that one pound, only to have it lost immediately after you stop drinking the stuff. My endocrinologist doesn't know what it is, neither does my general practitioner. Tapeworm? fast metabolism? God, I just want to look normal for once. In a society hellbent on losing weight, It really isn't easy to find someone who can help. Please help if you can