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, growing out mechanical and chemical damage = breakage and very high porosity. Past armpit length heading for waist.
Treatments: Komaza protein, Komaza Matani, coconut oil with sugar syrup. Hairveda Sitrinillah
Leave in: Fructis Sleek & Shine (old), Gliss ultimate volume
Styler: Umberto Giannini jelly, Boots curl creme, Rockin RInglets
Experimenting with: Mega-Tek, Curl Harmony reviver, Elvive Full Restore 5.