I'm just like you. It was painful putting on muscle, but it can be done. Your job literally needs to be eating. I used to go to the cafeteria on campus and eat 4 FULL meals a day. And by full I mean I would eat until I almost puked. Sometimes I did---but I would just swallow it back down. Can't afford to lose those calories, now can we?
On top of those meals I was having a weight gainer shake with ice cream and peanut butter. Some nights after hitting the gym I'd sit down with a spoon and a jar of peanut butter and kill 1/3 of it.
It's all about calories in versus calories out. Fast food will get you the weight, but it won't be the weight you want...so at least eat some good proteins and carbs too!
And this may seem counterintuitive, but if you aren't lifting weights, get in the gym. I can eat my ass off, but if my body doesn't need the weight, it won't take it. If I lift, I will put weight on faster. Don't go batshit crazy, but get some heavy weight on the bar and move some steel 2-3x per week. Notch back on the cardio a bit. And EAT!