When it is difficult to achieve and maintain flexion on your own it is helpful if the therapist applies a firm but gentle sustained force for twenty seconds at a time to help stretch the quads and to prevent the formation of sticky adhesions in the soft tissues around the knee, which could in time form into scar tissue and make the knee stiff.
You could get someone to help you with this at home. The force should not bring on excessve pain. After a minute of relaxation, the flexion can be applied again.
If this stretch is too uncomfortable, it may be useful to sit on a table with the legs dangling over the edge, and use the good leg to help push the problem leg into flexion, again maintaining the stretch for twenty seconds at a time.
Icing the knee will be helpful if the knee is painful and warm after these exercises.