It’s possible that your self-diagnosis is correct, but you didn’t indicate the usual behaviors of someone with anorexia. From your letter, it looks to me like you’ve done “everything” except the thing you need most to do — which is to consult with a doctor. You may need something as simple as reassurance that there is a wide range of body types and men develop muscle mass at a wide range of ages. But it’s also possible that there is something amiss with your metabolism that could be corrected with some medication.
Please don’t wait another minute to make an appointment with your physician. You’ve suffered with this far too long already.
I wish you well.