Look at some old sneakers. Are they more worn on the left on the outside?

Try better innersoles. There are many kinds, but it might be worth trying the kind that heat up and mold to your foot.

You also may need shim up the outside of the left shoe.

If your left leg is shorter, dropping the seatpost by 1 or 2mm is something else you can try. Mark the post so you can keep track of where you're at.