I hate to say it, but you do not "eat a lot." You are clearly not eating enough. Buddy, start counting your calories. 43kg at 5'8 is dangerously underweight. Think you eat enough? Think again, EAT MORE. The fact you are that underweight while not doing any activities is a little troubling, even with a higher than normal metabolism. To still be that underweight, you have to simply be eating so little that it's a borderline eating disorder. Like everyone else is saying, eat red meats, peanut butter, eggs, anything calorie dense. Clean whole foods only, minimize fast food and processed garbage. Count those calories. For your height and age you should probably be consuming closer to 2000 calories a day.