Varying motor speed with the L293D
Although I am still figuring things out, but i hope the following things help:

http://www.solar4rschools.org/sites/default/files/robotic-sunflower-lesson4-handout4.pdf

The problem, as far as I think is that the enable pins are the ones which are supposed to help you control the speed. Please connect them to the PWM ports on your Arduino/any other device you may be using for your project.

Or, you can simply write a code which keeps on disabling and enabling the enable pins to get the desired speed (that's what PWM is supposed to do but you can try it this way, at least).