The left masseter, if grossly imbalanced can be weakened with botulinum toxin. And yes, the frequency of injections will lessen over time due to reduced usage.
Do note that if you end up using the right side more because the left is weakened, you will end up with a strong right masseter, and need treatment for that too.
Some chewing equality and training is advisable, and I can guide you to the right direction for that if desired.
Hope it works out nicely for you!
