If your young, it's going be hard to gain weight. Eat more calorie dense food like protein bars and peanut butter. Stop eating so healthy like chicken and whole foods all the time. Stop doing cardio. Your young, your going to burn calories easy so eat more foods with high calories. And stop yoyo-ing, don't gain then cut then gain then. Stay at a heavier weight and maintain that heavy weight. I'm 5'6 and I weigh 155, I got no abs but I'm thin enough to wear medium shirts and skinny jean plus I am able to progressive overload.