Adjust H-bridge frequency

I am trying to run a brushed 12V DC motor off the H-bridge of my Amiga using the 12V port. Since my motor is brushed it does not contain a split ring commutator, and the frequency is dictated by the h-bridge.

My motor is not synchronising. How can I adjust the frequency of the H-bridge?

Furthermore, is there a recommended DC motor inductance range? This may also be causing me issues

Thanks in advance!