maintaining/rebuilding strength with injury

First off I apologize if this has been addressed in a previous post, and I realize getting medical advice over the internet is not the most efficient way to go. That being said, the past couple of months I've had some pain underneath my left kneecap on extension. Not sure what has caused this as I never had any moment where I thought I injured myself; it was one of those things that just gradually built up. I'm not currently able to do SLS PE1, not because of this issue but just because I'm not able to yet. I combine my foundation work with traditional weight-lifting and while I'm able to do barbell squats and single-leg leg presses without knee pain, weighted extensions are a problem. I don't want to just quit doing them, but I'm unable to work with the amount of weight I'm used to on my left side. I was wondering how exactly I could go about building the strength back up on that side. Do I do more sets or reps on that side, and gradually build up weight as the pain decreases? How are strength imbalances normally fixed?

Rule of thumb is to let your weaker side dictate the weight and volume and monitor yourself assiduously to ensure the stronger side doesn't take over in bilateral exercises. Form is always primary but should be 110% focus when dealing with imbalance to ensure the mechanics are kept perfect; there are elements of re education in addition to brute strength.

Your medical practitioner is the best person to ask