I play CM as well, and I've been told I'm a natural at the position.
Not very fast: Well, IMO, I don't think you have to be very fast to be an effective CM, but you have to be quick, first to the ball every time. You are the leader of the field, you have to be calling for the ball every time your team has possession. You should be running your ass off for that ball, fighting to win it back when your on defense, and organizing your attack. Quickness is much more important than speed here. But, it wouldn't hurt if you were faster, just saying.
Short: That's not exactly a disadvantage. Sure, you might not be able to compete with 50/50 balls in the air, but with a lot of muscle and your low center of gravity, you'd be surprised what somebody your size could do to a guy a foot taller than you.
Not the strongest guy: You're around the age where it's safe to start weightlifting. But, I'm no doctor, so maybe you should be asking your doctor that question. Nobody here on this forum can give you a weightlifting routine. Your routine should be based around your time commitment, and your level of fitness. Obviously if you've never lifted weights before, it'd be pretty stupid to give you a routine in which you have to lift 150 lbs. of weight thirty times in a row, right? Talk to your coach and he'll help you out on that.
Defending: I have the same problem. But, you can't be scared to put some body in those challenges. Remember, first to the ball, don't let him get the ball, if he does, don't let him turn, if he does that, don't let him pass the ball. Your job is to keep your man in front of you, and when he gets the ball, you need to put pressure on him like crazy. This will force him to (a) feel the pressure and make a mistake OR (b) pass it to another teammate. You've done your job just by making sure your man is in front of you and not on his way to goal.
Just believe in yourself. So what if you're short? So what if you're not that fast? Play to your abilities. Only you know what you are capable of, even then, you probably don't know what you're capable of. (Did that make sense?