I am 25 and feeling pain on the back side of my right leg knee since 2yrs. i can't standup for long time& i feel relax when i straighten my legs.

See an orthopedist: Having a problem for that long and which is apparently not getting better demands you getting to see your orthopedist, as this could be a meniscal/chondral lesion. Some bucket handle tears of the menisci do reduce spontaneously and you get to straighten your knee and feel better. You should get a MRI as well, but more importantly see your orthopod who can make the correct DX and will treat you.