Hello Lucy
It sounds like your hair is a bit over processed and has become totally brittle and excessively dry. I am going to recommend that you use from the Redken Smooth Lock range the shampoo and conditioner which will hydrate and smooth frizzy or wavy hair. Smooth Lock technology has a thermo-protective polymer in it that forms a humidity resistant, heat activated barrier which will lock in moisture and lock out frizz. In addition, it contains almond oil for nourishment and IPN, which reinforces the hair. Once every third shampoo, instead of conditioning, treat your hair after shampooing with Smooth Lock Buttersilk Masque for a more intensive hydration and nourishment. After towel drying each time you have shampooed, apply Redken Smooth Lock Stay Sleek, which lasts through 3 shampoos to keep hair sleek and frizz free. This product is designed to work with the heat of a flat iron. I would also recommend 6 weekly trims just to remove the split ends over time.
Warm regards
Heidi