Eat when hungry or force myself to eat every calorie__CONFUSED



I've been doing FB Mass for 3 weeks bc I want to extend beyond toning to build muscle size. I've been reading over and over that you have to increase your calorie intake in order to grow. However, I'm not hungry enough to eat more than 1300-1500 calories/day. Which is what I've calculated (through websites) that I need per day in order to gain size. Even 1500 is a stretch for me to eat, though. In fact, when I do force my self to eat the extra calories, I feel sluggish. Prior to this, I only ate when hungry, planning my meals according to an intermittent fasting schedule.

What do ya'll think? And what have you experienced in gaining muscle and calorie intake? I'm trying to not be obsessive about it, but I'd like to get a clear answer moving forward bc this 200 calorie increase is not working for me. HELP!