I just installed a new replacement motor and pump (1.5 hp, 230V, 2speed). It did not come with a wiring diagram. When I turn it on, it shuts down within a couple of seconds by tripping the breaker switch on the main panel. The GFI is not tripped.

I assume that I have plugged the wires into the wrong terminals on the motor. How can I determine which terminals to use with the red. black and white wires?

send photo of terminals and what markings they have against them

you may see L(low), H (high), C(common)

white to common or neutral, red to high, black to low (in some cases red and black are opposite)

My terminals are:

COMM L1
L2
HiSpeed

I've connected white to COMM L1, black to L2 and red to HiSpeed.

My problem now is the motor stays at hispeed and does not shift to low speed.... any suggestions?

to prevent any further possible damage - employ a tech
its possibly not wired right, but you may have damaged or fused a pcb relay controlling the pump