If you got a trainer and bike computer with cadence, I'd try the following.

Pedal with one leg at a time. Pick a gear/resistance which limits each leg to 3-5 minutes. Make sure you pedal each leg at the same cadence. Find out which leg is stronger.

If you left leg is weaker, you may want to do some training with just the left leg.

If you left leg is stronger (or both legs the same), you are working your left leg harder while cycling for some reason. Check you alignment (i.e. shoulders and waist square with bike, both knees tracking the same, etc.)