Your best solution would be to convert the circuit over to 230V assuming only the pump is on the breaker. Changing it to 230V will cut the amp draw in half and allow for the 12AWG wire to carry the load. You could increase the breaker rating but the wire is to small for the load and keeping it will lead to future problems if you leave the circuit as 115VAC.
Answers & Comments
Your best solution would be to convert the circuit over to 230V assuming only the pump is on the breaker. Changing it to 230V will cut the amp draw in half and allow for the 12AWG wire to carry the load. You could increase the breaker rating but the wire is to small for the load and keeping it will lead to future problems if you leave the circuit as 115VAC.
John