When I camp out over cold weather using a tarp, bring the sides right down to the floor pile uop/ fill in any gaps at the bottom, leave any leaf litter on the floor, cover the floor with some ground sheet to try to stop the damp from the ground rising,
Pull the ends in on the Tarp and boritto it, so its enclosed, I have even run a petrol/tilly lamp in inside in relay cold and damp weather to try to dry things out, but only do this if you can get GOOD VENTILATION....!!!!!! _ could use a UCO type lantern as well
Keep things as dry as possible as well, To try to help keep you sack dry warm, I have even taken a hot water bottle, and re heated the old water for a couple of nights... leave your boots at the door way
Last thing at night boil some water, warm up a flask, then put some coffee in it... the you got a warm drink as soon as you wake up..... then just keep it full trough out the day, always good to have a hot drink to hand... remember try to keep your mood up beat... it helps to do things and do them first time.