I second what Richard said.
You are smoking your diodes because they are under-rated (all the kinetic energy in the motor will be dissipated in the diode, windings and mechanical losses, which can be quite a bit of energy) and using an open-drain N-MOS is the preferred way of building switched circuits since common-source makes the gate much easier to drive: in many cases, you can drive it directly with 2.5-3.3V logic.
The situation when you turn off power to the motor is actually worse than what Richard said: when you apply power to the motor, your current is: (source voltage - back-EMF - brush losses) / winding resistance but when you cut power, your current becomes (back-EMF - brush loss - diode) / winding resistance... if your back-EMF is 8V, your brush loss is 2V, diode is 1V and your winding is 0.2 ohms, your diode ends up seeing 25A during electro-braking instead of the 5A you might be driving your motor with at 12V.