Your metabolism is slow just out of being underweight. All underweight people who are restricting food have slowed metabolisms, because the body is trying to conserve calories and burn through food slower.
If you had an adequate amount of weight on your body, you’d be able to eat without seeing weight fluctuations. Your body wouldn’t be slowing down your digestion and trying to squeeze every last calorie out of the food.
You feel cold also because your metabolism is slowed. If your metabolism was faster, you’d feel warmer.
You know why you feel weak and fatigued and want to sleep all the time... Wanting to sleep = metabolism slowing down even more... Your body is literally trying to slow you down, force you to rest.
Your metabolism will slow down even more if you stay at your current low weight. It does so in an effort to make you gain weight. Your metabolism will keep making it easier and easier for you to gain weight, if you restrict. It'll keep slowing down.
Your weight will not magically go up without you eating more calories. Energy intake (calories eaten) is what determines weight the most. You can't eat your current intake and have your weight increase as your metabolism slows. What will happen as your metabolism slows is you'll notice yourself getting colder, having more digestive issues, hair falling out, other things etc. The body will stop spending as much energy on non-essential systems.
Your hair and nails will fall out. That happened to Chef AJ when she was anorexic. I say “will” because if you starve yourself forever you will end up losing everything, including your life.
Thinking about it from a scientific perspective, your body will NOT increase metabolism (to burn through food better) unless it feels it has a surplus of calories. So if you’re restricting in any way, your body will NOT increase your metabolism.
Your body WILL increase metabolism if you eat a LOT.
Because then your body will go “Oh! I suddenly have food! And I’ve had a lot of food for weeks! I can finally turn up the metabolism because I need to process all this food."
If you eat more than usual for just 1 week, that’s not enough. You need to eat more for a period of time for your body to get the memo that you have adequate food. Your body has spent X years thinking it's in a famine.
It's not going to decide that you’re feeding it enough, until you nourish it adequately for a period of time.
What the metabolism is like in recovery
Now that I'm recovered, I can eat whole, natural foods ad libitum while maintaining my healthy weight for me. My body isn’t like yours, slowing its metabolism down because it thinks it’s in a famine. I never go hungry for too long because I eat immediately. So my body feels safe - it doesn’t have to turn every calorie I eat into body fat, like your body has to right now in order to survive. My body knows I am in a land of plenty, an environment with enough food, so it doesn’t need to convert every single calorie into body mass.