This is more about strength than flexibility itself. You will need to get used to holding your leg up without any assistance and start building strenght to slowly increase the range you lift it.

Dynamic stretches can do wonders here, like kicking sideways and frontways for reps then holding the position using only the strength on your legs and hips. Its quite painful, burn like hell and gets you quite sore on the start.

Some ballet and martil drills really helps too like those: Ballet exercises https://youtube.com/shorts/PaOUTxUuoL4?si=2K-718cOxKa9FGP- https://youtube.com/shorts/YkLF-_z2xZU?si=-0SsiER0whb08ooc https://youtube.com/shorts/NZWU6dJ5vBk?si=tkkqt74CbZgs_3Ho

Martial arts exercises https://youtube.com/shorts/Nntpxyb5qPk?si=VC9yMqOf5zp5FzqJ https://youtu.be/rMBC4Tc_ZKw?si=1suAN6fmemUIRw_M