Starter not disengaging?

It sounds like your wiring path is wrong and you are powering the SOLENOID through the key switch RUN position, keeping the starter engaged.
12 volts from the key switch spring loaded START position should run through the neutral safety switch to the RELAY, and then to the solenoid which engages the starter.
The run position of the key switch should not have any power connection to the starter or solenoid.