Don't listen to Juve, I'm 5'10'' and 59kg and I'm 15, I'm a great DMF (me being full of myself and all
). Anyways, from your description though, you and I play different styles. I guess you could define DMF as two roles: either the stopper, or the deep lying playmaker. Your description is leaning more toward the stopper side (with mine being the other one).
Basically, in order to take advantage of your skills, you need to be up and around the field at all times. You sound like a hard worker, one that can keep going for 90 minutes non-stop. As a stopper, your job is to always, i guess "stop" the play, or halt their attack. This applies to both roles of DMF because it's called a "holding midfield".
So when you defend, basically lure their attacker toward you, and keep backing up but let him gain ground on you (never leave the attacker around 1 m in front of you because that's usually the point where he/she makes their move). Always keep your head up, know the game (you can improve that by watching games and reading articles such as the ones we have in this site). You need to slow them down, so stay jockeying but don't be aggressive and attack the man or else he'll pass it off and the attack will continue. Only use your speed from you have to, such as when you're switching from one side to another or you run down a speedy attacker.
As for strength, we skinny people have to know that many wingers/strikers can't use their bodies as effectively as we do. We always go inbetween the ball, and use our speed to reach 50/50 balls first. Co-ordinate your defense with your defenders, and you should become a relative threat.
Also, on the attack, the stopper usually acts as a transition between defense and midfield. Keep your plays simple, as you still reside within a dangerous area of the pitch. More than likely, your first option when you get the ball is to turn and pass it either back to the defense or to the sides for the midfield (because you don't usually get the ball sent back to you unless your midfielders/forwards are being pressured badly). Your role in attack is to spread the play without losing your position.
and Rulezzz is just making a lame excuse because he knows he has no advice to give