Badly need advice :(

I've tried several different hair products (Tresseme, Neutragena Triple Moisture, Aussie, Charles Worthingtons, Great Clips, Head&Shoulders when I had dandruff, and quite a few in the past that I can't really remember)
I've tried different kinds of brushes and combs, and some smoothing/moisturizing leave in conditioners...

I've done google searches for hair advice, and now I turn to this forum. XD;

I have very very ultra fine hair that I can't seem to bulk up, and it knots extremely easily. I'm talking just shifting it over my shoulder and there's a rat's nest. I've asked at salons I go to whenever I get my split ends trimmed (which I do regularly), but they've never really given me proper answers. My hair is also extremely dry and frazzled even though I shampoo only once in the shower and ALWAYS condition, and I'm always sure to use moisturizing products.
Admittedly, I do use a flat iron, but I use it sparingly and not every day. I also use a protective serum before use, and put in some hair softening leave-in conditioner when I'm done.
I don't excessively use hair-sprays.

I brush my hair often; I brush now and then throughout the day, before I take a shower and after, and before I go to bed.