It's a deceptively difficult position to play I think. I'm a winger and occasional full back, if your not really used to playing there then positional wise it's difficult, you find yourself not knowing weather to come across, who to pick up, weather to follow 'your man' if he comes inside or stick to your position etc.
Other than what's been suggested, talk to the lad playing infront of you, get at him to track back and pick up the winger as much as possible, meaning you can tuck inside and pick up a forward or if he's free pick him up and let you centre half know. Play to your attributes too, if you can, stand off your man or show him down the line and dont commit yourself or dive in. You dont sound too confident either, try to get in the mindset that nobody is going to get by you. If your not overly confident , keep it simple too, until your comfortable.
Obviously I've never seen you play but as someone who sometimes at full back despite not being the best there, thats the type of things i try to tell my self.
