L298 overheating issue
You haven't labelled + and - on your battery!

Assuming that battery pin 1 is positive, because it's connected to VS of the L298, then your diodes are the wrong way round in the schematic.

Edit: the 0.9V is exactly as expected, as that's the forward voltage of the diode. You're sinking the full current of your power supply through a diode and the L298.