If you find yourself on a very cold night and your sleeping bag is not up to the challenge, there are things you can do rather than freeze.

Put a waterproof bag around the foot of the bag and stuff it with leaves, pine straw or extra clothes.

Increase the insulation under you with leaves and/or pine straw.

A windproof/waterproof layer, such as a bivy bag, trash bag, poncho or tarp around the outside will slow heat loss.

If you have a blanket or poncho liner in addition to your bag, you can use it inside the bag or pin it in a bag around your bag to increase insulation.

Wear warm clothing inside the bag, especially extra socks. Clean clothes are more insulating than dirty ones.

Camp site selection can help. Camp on the south side of hills that have been in the sun most of the day. The ground and rocks will radiate heat for a time. Camp out of the wind. Do not camp in low ground and close to water, cold air sinks and the water will cool things a bit. Camp part way up the sides of hills above the cold air layer in the valley.

Letting your sleeping bag air out during the day in the sun will aid in keeping it dry and maintain the loft in the insulation. Being stuffed tightly in a bag after much use is not good for it.

Have some pajamas, like Hikermor said. A good base layer used only for sleeping, kept dry and clean, with warm socks and a hat will help a lot.