Welcome! Sometimes old hair is just too damaged from past abuse to be salvageable and needs to be cut off, hair is dead so can only be temporarily repaired. Which deep conditioner and leave in are you using?
Are you being SUPER gentle with your twists? I don't really understand the concept of protective styling, it's all manipulation and forcing the hair into an unnatural shape which potentially stresses the hair and causes mechanical damage.
Do you know your hair properties, and have you considered having a hair analysis? Komaza Care are very detailed, will tell you what type of damage you have and how far up the hair it is.
Komaza Care HAIR ANALYSIS Service - Rave! - Long Hair Care Forum
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.
CO-wash: Inecto coconut
Treatments: Komaza Matani, coconut oil, Hairveda Sitrinillah
Leave in: Fructis Sleek & Shine (old), Gliss ultimate volume, Inecto argan
Styler: Umberto Giannini jelly, Boots Essentials gel
Experimenting with: going back to basics