I would advice you to use some external supports (Mobile Knee Brace), to counter the knee
instability, if you are facing presently.
The cause of your pain is joint arthritis and cartilage
damage. So options at present are-
1) Inj hyaluronidase, etc like stuffs, which are supposed to aid in cartilage growth.
2) Cartilage regeneration.
Beside these you don't have much options left.I am strictly against joint replacement as you are too young to undergo this procedure.