Some people are just naturally slightly underweight. I was for years. Now I'm considered "normal" but still on the very light side, and I believe any of the weight I have is actually muscle because I've started dancing and working out. My fiance is also on the skinny side; I believe he's considered under weight.

First, I don't think you should stress about this too much. He has told you that he loves you the way you are. Men like women that are confident in themselves and about their bodies. Working on your self esteem will help ease the worries that you have.

Second, make sure you're eating healthy, getting enough calories (good calories, not empty ones that you find in junk food and sodas), and working out (you can do work outs like Pilates that will help you get long lean muscles as opposed to bulky muscles). That way, regardless of how much you weigh, you're healthy, and that's what counts.

He's dating you for other things than the number on the scale. Try to stay positive and remind yourself of all the things he loves about you, your personality, sense of humor, your gorgeous smile. Don't be too hard on yourself

Take care