How can I change the way my hair is? help?

heyy, I need some help because my hair is just so crazy . Okay so my hair is naturally loose curls, dark brown. and about 4 inches under my shoulders . And it's just so brittle, like I tie it up into a messy bun, and when u take the elastic out .. my hair just stays in place, its not loose or whatever, it's so hard to manage. it's like it doesn't even go into a messy bun, it's just goes into a brittle bun. And I hate it, I have great shampoos and everything. But when I come out of the shower. I put in some leave-in conditioner, and I comb through my hair while it's wet, and get it all de tangled. and theres still nice curls . & then I let it air dry.. and "BOOM" so brittle and puffy !! please tell me some ways on how to makes it more manage able and not brittle, that will just fall out of my messy bun when i take out the elastic. Please help, thanks :) ( and I hardly ever straighten my hair ) .