How can I prevent this hair texture ?

So I have fine hair and mostly flat hair (no volume). My hair always looks like that in the picture if I didn’t put heat on it. Air drying and also today I slept with a bun which resulted in some wavy hair and I needed to brush it out so now I look like Hagrid the whole day.

Idk what to do I don’t believe that I some other hair than straight hair. When I was little I had super straight hair but now I’m not sure anymore. The thing my hair cannot hold that much product. I already use a conditioner by kerastase it’s the orange kind.

What else can i do ? I can’t put heat on my hair forever.