In reply to johnlc:
All a sleeping bag does is slow down the leaking away of heat from your body. If you're feeling cold you are not producing heat greater than your bags ability to retain it.
Eat more is the easy bit. More kcals, ideally of slow burn stuff rather than sugar, will metabolize into body heat. This changes with age and other bodily changes.
Sleeping bags work best the more stable an environment they are in, ie still and dry. If you've minimized heat loss into the cold ground with a mat (by far the biggest source of heat loss), then reducing air movement around the bag with a tent or bivy bag, and maximally lofting/drying the bags insulation with body heat optimizes those factors.
Get these things right and you can sleep in surprisingly adverse conditions, open bivys on ledges etc. I like to get into my bag asap, clothed, eat, and as my body warms open zips or remove clothing so body heat gets into the bag itself. In a tent, I have a few blasts of the stove (I know about the risks of using a stove in a tent) to rise the temperature inside, which draws the condensation point inside the bags insulation out beyond it.
Before you go looking at expensive new bags, have a look at a cheap summer weight synthetic sized to go over the bags you already have. For minimal cost and weight you will get a lot more function than yet another big bag.
And get a good piss bottle. Keeping a liter of liquid warm when you don't have to is inefficient and no point harnessing all that body heat only to get out for a piss and lose it all.