You have Medial Plantar Nerve entrapment. See the diagrams below showing the nerve distribution of the sole. This nerve serves the first three toes and the ball of the foot.
Contrary to the suggestions from others above for you to get more arch support, this condition is caused by having too much arch support. This nerve passes along the bottom of the arch. Too much arch support puts pressure on this nerve, causing numbness in the areas served by this nerve.
The reason you have the problem only on your left foot is because your left foot is flatter than your right foot. Having flat feet makes this problem worse, because a flat foot causes the bottom of your sole to make heavier contact with the shoe, pinching the nerve.
To solve this problem, buy a pair of shoes with the least amount of arch support possible. Dead flat insoles. You could also try cutting a hole in the arch area of a pair of insoles, and then putting them into your current shoes, to cancel out the raised arch area built into the shoes.
How I know all this: had the exact same problem. Solved it this way. For me it was the right foot.
With all respect, how could you possibly know if the OP has high or low arches? This could certainly be the problem, but it could also be the problem I have seen many times from a lack of support. So how do you know it is one and not the other?
It is not hard to tell what kind of arches you have.