You need to get a specialist hair product as it sounds like you have some allergies to what you have been using
If you are tired most of time and losing hair, I'm afraid you may have anemia or close to it!
And if you have been neglecting your nutritional needs, it is just a matter of time until you develop deficiencies, which lead to potentially serious health issues.
Hypothyroidism & hypoglycemia should be also ruled out.
The first thing I would recommend to you is to start immediately, like in right now to optimize your nutritional intake.
No skipping meals, no junk food ! And try to reduce your stress.
Stress constricts hair shafts
The appointment with the dermatologist is not going to bring resolve to all your problems, if you don't support your body by supplying vital nutrients to maintain, repair, heal regenerate & and all the other functions it carries on all the time!
I don't see as a primarily dermatology issue, according to what you posted,
but more of a nutritional and dietary issue.
If you have an unaddressed eating disorder, digestive or G/I issues,
it is now time to pursue this.
So the second thing I recommend, is make an appointment with your PCP
discuss all this and also make sure you get a complete nutritional & iron panel (blood tests) to be checked for deficiencies asap!
This is not rocket science, it is basic health science and if you pay as much attention to your nutrition, as you do to your hair, chances are that everything will improve gradually and you won't have to worry about losing
more hair in the future (barring the possibility that all this has already resulted in some irreversible damage, as I do not have a complete clinical picture other and than what you're posting here).
I hope this helps, however, my suggestions and comments are not intended to replace medical advice.
Best wishes,
Niko
PS.
My intention is not to pick on your eating habits, but to make you aware
that your suspicion "I thought maybe a vitamin deficiency" is very likely
and that you should take the necessary steps to improve your nutrition.
BTW 50 to 100, is the number of hair we naturally lose each day, so I don't know if you exceed this range, but stress, low thyroid,anemia and deficiencies are factors in the higher or above range hair loss.