1-Volt at 1.5-Amps = 1.5-Watts of HEAT that must be dissipated.
12-Volts at 1.5-Amps = 18-Watts of HEAT that must be dissipated.
12-Volts at 0.5-Amps = 5-Watts of HEAT that must be dissipated. ( probable start-up-Current for your Motor )
12-Volts at 0.25-Amps = 3-Watts of HEAT that must be dissipated. ( maximum continuous-Current allowed for your Motor )
Does your Regulator have a Heat-Sink that can easily dissipate at least ~5-Watts of HEAT ?
A TO-220 Transistor-Package,
with no Heat-Sink,
can "usually" dissipate around ~1-Watt of HEAT,
before getting TOO-HOT and smoking the device inside.

Your Regulator will automatically shut-down if it gets too HOT, ( if you're lucky ).

Please describe the Mechanical-Load that your Gear-Motor is having to accelerate,
and any drag that it must overcome.
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Or maybe better yet, try improving your heatsink or even better power your 7812 from 15 volts and see if that solves your problem.