Fix hair damage?

I have fizzy, thick, long, black hair and I flat iron my hair every other day and I wash my hair every other day, but after the shower when I leave my hair all nautral, my hair feels all hard and not as soft like it is when I flat iron it. What do I use...

to protect my hair from damage?

I have hair like you aswell, it took me a long time trying all sorts of products to suit my hair, in the end I found having the right shampoo and conditioner is the key.
I now use 'Herbal Essences Seductively Straight' shampoo and conditioner.

I usually shampoo my hair twice during each wash making sure its very lathery and bubbly, then I put LOTS of conditioner on leaving it for a couple of mins then combing it through before washing it out.
I let my hair dry naturally as I find a hair dryer makes it very frizzy.
I also only wash my hair every 3-4 days, I know it sounds a long time but my hair is so thick it takes ages before it starts to go greasy.

Hope this helps, it works for me

Stop straightening your hair so often - that is /frying/ it.

You could use a quality serum and anti-breakage system (such as Tresemme's).