It sounds like you use alot of heat, and the problem with heat is that it can be a double-edged sword. To a point, it can give you soft swingy hair. But if you overuse it, it can make your hair stiff...believe me, I know. When I was using my curling iron really often, I could make my head look like:
with no holding product.
So my advice to you would be to put down the heat. No more blow-drying, no more flat ironing. I think that by using less heat, and raising the moisture level of your hair through intensive deep conditioning treatments and daily moisturizing, you'll see softer, more pliable, and most importantly, healthier hair over time.
If you want to wear your hair straight, then try rollersetting and air-drying, or drying under a hood dryer.
Or you could just experiment with different looks, instead of just wearing it pin-straight all the time. I know you like it straight...I too enjoy some straight swangy hair every now and again. But our hair just wasn't meant to be that straight all the time. And when we try to force it to be that way, it fights back. So don't be afraid of texture and non-bone straightness! Try air drying your hair and then pin-curling it for bump, or wearing braidouts, twistouts, or bantu-knot outs. These methods also have the added benefit of making your hair look and feel fuller. I have fine and sparse hair, and when I want it to look fuller, I rollerset it and air dry. Bam, it's like double the follicles.
And finally, I'd encourage you to try to space your relaxers further apart. Even if your stylist is only doing new growth, you're still exposing your scalp and hair to a caustic and dangerous chemical. It stands to reason that if you're doing that less often, it'll be healthier for your hair and scalp in the long run.