Why does my hair keep breaking off?

The last time I used any heat tools (flat iron & blow dryer) was more than a year ago, and I've had my hair cut and trimmed since then. I don't dye or chemically treat my hair in any way. I don't tease my hair, use styling products, or tie my hair in tight ponytails. I shampoo and condition my hair everyday and use a deep hair masque once or twice a week. I also use Chi Silk Infusion after each wash while my hair is still damp. I always towel-dry my hair and only use a wide-toothed comb to comb my hair once it's dry. Somehow I still always get split ends so I assumed it was because I don't my hair trimmed often enough. I used to wait several months to a year to get a trim...so I recently got a trim about 1-2 weeks ago to remove all my split ends. My hair felt nice and healthy again without all the split ends, but now I've noticed that I have A LOT of split ends again. My hair is tangling constantly and breaking, and it's really troublesome to untangle the knots. My ends are still really dry even when I condition them. I've used so many different brands to see if one works better than the other, but none have improved my hair's condition. I've been taking multivitamins and Nioxin's intensive therapy recharging complex, but they don't seem to be helping either. I really don't know what's going on. Please help.