How can I fix my dry/damaged/unmanageable hair?

My hair is so dry and gross no matter what I do it is so unmanageable and gross. I'm 16, caucasian, and so are both my parents as well as my grandparents so I know it's not genetics that causes my hair to be the way it is. I used to have beautiful hair when I was younger (age 1 to 12) then I started dying it and stuff. BUT i havent died my hair in almost a year now I get trims regularly and I barely even style it with heat anymore. No matter what I do I have like one good hair day every 2 months! My hair on the front of my head where my bangs are, is thinning and I have no idea why! I take a multivitamin every day (GNC Active Womens) and I sometimes skip days from washing my hair to let the oils in my hair help it or whatever. I trim my own hair with regular cutting scissors...but theres no way thats the problem because even my roots are gross. Roots should always be the healthiest part of your hair because it's the newest but mine totally isn't! I'm thinking about switching my shampoos to more expensive ones like instead of herbal essences I'll switch to Nexxus or something. I switch my shampoos after the bottles empty to prevent buildup which is what i read in a magazine but I seriously have no idea why my hair is so gross no matter what I do to it! Im thinking about investing in the Wen hair care system as well so maybe thats a good place to start? I dont know! Any tips on vitamins to take, hair products, etc I'm totally open to! Thanks so much everyone!