I agree substantially with most of the suggestions made so far. I believe that I am the one who sleeps in a sleeping bag on a hard floor for a few nights before every backpacking trip. That kind of keeps my back in shape.
1. There is an old Sierra Club saying that if your feet are cold, then put on a warm hat. If you are wearing a hat, then try a warmer one or else two. One snow camping trip leader I know wears two thick wool hats in cold weather. It looks kind of funny, but it gets him excellent results. For backpacking purposes, I would suggest using one thin stretchy synthetic beanie with one thick warm one over that. That way, you could split them and use just one at a time for milder weather. If you don't need the second hat, then stick it inside a stuff sack that holds your "pillow." If you don't have enough lightweight stuff to stick in to make the pillow, then carry along a few square feet of bubble-pak (the plastic bubble sheet that delicate stuff is shipped in). It weighs almost nothing.
2. I don't know your sleeping mat. However, for most sleeping bags, if the temperature rating is good but it still seems cold, then the sleeping mat may be part of the problem. You might put some closed-cell foam mat in there and see if it helps.
3. The food might be a problem. You have to carry food that you will eat, and you ought to be able to find food to carry that you can eat under any circumstances. One friend of mine taught college nutrition, and she always organized evening meals that were thermally hot as well as spicy hot. If that includes something like meat with a little fat in it, then that takes a while to digest, and that kind of helps keep you warm all night. Of course, plenty of other hot food and drinks are important also. Lots of people do not want to drink liquids in the evening so that they will not have to get up during the middle of the night. Well, OK. You decide which is worse.
4. Yes, minor altitude sickness can inhibit appetite. But there are several other symptoms as well. You ought to be able to sort that out.
5. I saw one backpacker who was super low on energy, could not carry her own pack uphill, had no appetite for dinner, felt super cold, etc. It turned out that she had been on a fasting diet for three days! Geez. Once we got a couple of hot meals into her, she came back alive.
6. In general, it is thought that vapor barriers are effective for cold weather sleeping, but mostly when the temperature is seriously cold, like -10F or 0F. In Northern California I never see anybody using a vapor barrier any time when it is well above 0 F.