So the prior electronics only allow forward motion. Here is a super quick design for a circuit that uses two blocks, one for "Enable" and another for "Forward / Reverse". The Enable line can't really be analog, but it can be PWMd through a few different levels at least. 

You can play with it at:
https://www.tinkercad.com/things/9NsnYvF5ILO-light-block-h-bridge

by clicking Run Simulation in the upper right corner and then clicking on the opto sensors and dragging the sliders around. The position of the sliders is a bit... tricky... but you can figure it out. 

One of these days I might actually build it in the real world. I need something like this the play with my twisted string actuators.