Dont drink for a couple of hours before bed. Have an empty water bottle near by. Practice...The deed...before you commit

I understand this is a 2 month old post now so probably of little use to you, but for the benefit of others:

When sleeping outside the ground is your enemy. Your bag may be rated to forecasted temperatures but if you don't have an interface between your bag and the ground then you're in for a cold night. There are several options, from camping industry favourites like air and foam mats, to animal skins, to simple blankets, to cardboard, to a thick layer of vegetation. Anything that will stop your body heat leeching into the cold ground all night.

Air mats are the current trendy favourite but I wouldn't personally recommend them. They're noisy when you move, a puncture would ruin your night, and they don't provide any more comfort than a foam mat. Foam mats come in various thicknesses, and reputable ones will have ratings similar to sleeping bags. Thicker is usually comfier and warmer of course, but that comes with additional bulk and weight to carry.

Location is also important. You may look at the forecast and it says 0°C but that's based upon data from a weather station which is usually up in the air. You'll be lying on the ground where temperatures will be lower. Particularly if you're in a dip or at water level. Try to balance being as elevated as you can with being sheltered from the wind.

Never sleep with the same socks you've worn all day. They may feel dry to the touch but they've absorbed your sweat throughout the day. Keeping your feet warm is essential to staying warm. Put fresh dry socks on before bed, or if that's not possible it's better to wear no socks than worn ones.

Mountaineering: The Freedom of the Hills is an incredible resource if you wish to learn more