One thing that you might try doing is drinking cranberry juice every morning. My father had to do that for 7 years while on kidney dialysis and even living in Michigan with it's winters, he never caught a cold in those 7 years!! The tiredness you experience is probably from a simple lack of sleep; teens don't get anywhere near the sleep that they need for their growing bodies. Muscle is repaired during sleep. You need at least 8-9 hours of good sleep every night.
Hey,
I'm not sure how much HELP i can be but i can tell you this, that sounds more or less like my time in school. The only difference is i am and was underweight, and i didn't see that many doctors etc..
All through middle and upper school i was almost constantly ill, caught every bug that went round, even got to school one day and collapsed in the playground from nausea before school had even started and had to get picked up to go home, i spent maybe 30% of days that i WAS there waiting in reception to be picked up because i felt really sick or had a migraine. And on top of all that i was drained of energy and tired all the time.
Everyone i spoke to put the tiredness down to growing/puberty because i was the tallest kid in my year always, and now i'm 6 foot 7. Your tiredness may be down to the same thing or it may also be down to expending all your energy on recovering from an illness just in time to catch the next.
I'll tell you this, after being ill so much at that age, i've spent the past 5 years hardly EVER getting ill, i don't remember the last time i had a cold or flu real bad even when i spend alot of time around someone who has a cold. This is because all these colds and illnesses build up your immune system once you've recovered from them and i'm pretty sure you can't catch the same "cold" twice.
I don't know what to suggest on an immediate "cure" but i've told you this as to give you hope that one day your immune system should be able to fight off bugs before you even get the first symptom. That is of course if your just going through the same thing i did.
And i know how hard it is to focus on/complete school work when you always feel like that.
If you ever want to talk about coping with it then feel free to message me. I hope you find a proper answer, preferably from a professional, soon because i know how hard it is.