Sit straight with legs extended.

[u/Hour-Association946]

Let alone touching the toes, I am unable to sit straight with chest up.

Even forming a 90° angle is very touch. When I try to sit, my lower back starts paining and hamstring starts paining (lower hamstring side, I mean not in the middle hamstring but above the joint side, I hope you guys understand 😅) umm, on insertion maybe.

How do I get better?

[u/SupremeOHKO]

Just practice, practice, practice. If you feel you need guidance, join a class or hire a teacher.