Maddie, do you have a good diet? Make sure you are getting enough iron in your diet as low iron can cause these symptoms - although if this was the case it should have been picked up in your bloods. But just make sure you eat plenty of fresh food and drink at least 1.5L of water a day - I know I feel exhausted when I've had a week of binging on bad food and don't drink enough water!
You can also get to the point where you get too much sleep rather than not enough. 15 hours is too much sleep and you are just about guaranteed to feel tired after that, as silly as it sounds. Try going to bed at 9-9.30pm every night, and setting your alarm for 7.30-8am every morning. That gives you more than enough sleep, and also gets your body clock back to normal after a week or so, and usually you'll feel much more energised.
Make sure you get outdoors, just go and brush Chinga for a while, or even better go for a 30minute walk. Fresh air is fantastic!! Sitting inside all day 'resting' will make you even more tired!
Keep yourself busy with things to do, nothing strenuous, but just so you're not sitting in front of the computer all day or watching TV - again that will just make you tired! If you can't concentrate on something, get up and walk around for a few minutes then try to come back to it. Maybe do the task outside in the light and sunshine rather than under artificial house lights.
It really does sound like an iron deficiency to me, the dizzy spells, fainting and fatigue are all classic signs of not enough iron, I hope the DR is going to get a second lot of bloods done to double check, as often a first lot does not get a result back.