Motor Current Limiting Algorithm
Provided that you just want current limiting and don't need motor short-circuit protection, you could implement cycle-by-cycle current limiting (also called hiccup-mode current limiting).  

Instead of trying to do full loop closure on the current, simply use a comparator to turn the H-bridge off as soon as the instantaneous current crosses a pre-set threshold.  It can be implemented as a comparator in either hardware or (fast) firmware.  The H-bridge stays off for the rest of the cycle, but is allowed to turn on with the next cycle.  Note that this approach requires an inductive load to work effectively.