Let's make the assumption that the motor is a small tiny, and wide piece of wire (more like a slice of wire).
It's resistance is (lets assume) 1 ohm.
Let's add a resistor in series, to make the load bigger than 1 ohm.
This will also drop the current requirements, and just maybe then can you get away with your transistors.
and why are the diodes connected between emitter and collector of the transistors?
And if you want an equation, use this:
Motor current consumption = (Supply Voltage) / (Motor's internal resistance + resistor you want to put in series with it).
Sure, you might notice a loss in speed, but it is well worth the money, and time.
From the top of my head, the resistor you should use should be at least 1/2 to 1 watt resistor with a value between 1 and 50 ohms.