Welcome! Heat damage can make hair straight, it won't always recover. So can wet brushing and ponytailing, both can cause substantial mechanical damage but usually some of the curl will return once you stop. What type of brush are you using?

Don't you use conditioner daily? The ingredients in these attract and hold water, patch damaged areas, help protect from the friction of the brush. Basics are cationic surfactants and fatty alcohols, but ceramides, panthenol and hydrolysed protein are beneficial to damaged hair.

You might blend your oil with your ecostyler to make a custard
Eco Styler Olive Oil Custard - YouTube
2a-2c, medium texture, porous/ colour treated. Three years CG. Past bra strap length heading for waist.

CO-wash: Inecto coconut/ Elvive Volume Collagen
Treatments: Komaza Care Matani, coconut/ sweet almond/ fractionated coconut oils, Hairveda Sitrinillah
Leave in: Fructis Sleek & Shine (old), Gliss Ultimate Volume, various Elvive
Styler: Umberto Giannini jelly, Au Naturale styling gelee
Flour sack towel, pixie diffuse or air dry.
Experimenting with: benign neglect