Here are some things that could be happening:

Not enough protein in diet. Hair needs protein in sufficient amounts to grow.. we aren't talking a protein pack for the hair. Eating meats, or nuts may help somewhat with the shed. Even if you are eatin "some" meat, if your diet is primarily grains, fruits and vegetables you may have a weak protein source. If you do not like meat, there are plenty of protein alternatives at health food stores to help make up the difference.

Gelatine. Trying gelatine in your diet may work wonders as well. Gelatine (even flavored yummy gelatine desserts) have been proven time and time again for strong nail and hair
growth.

You may be sensitive to chemicals in commercial shampoos. This is the one I think is the problem, mainly because you have said you have always had this problem. Instead of using your shampoo, get a conditioner and try conditioner only washing for a week and see it it improves.