Try using a smaller amount of coffee solution at a time. Instead of 4 or 5 cups all at once do a 2 cup coffee enema, hold for 10-15 minutes then evacuate. After you have finished with the first elimination do another round and repeat. Also check the temperature of the coffee solution. If it is too hot you won't be able to hold it and if it is too cold it won't be of much benefit. Normally it should be 98-105. (I got 95% of this information here at CureZone! The rest was from books on detoxing)
I have been doing Coffee-Enema daily for the last couple of weeks and it gets easier each time. I find it most effective in the morning after my regular bowels movements AND a pre-cleanse plain water enema. Do the pre-cleanse even if you just went, it might help you more. Of course you must do a Coffee-Enema on an empty stomach. Having any food in there other than water and maybe some chlorella to absorb the toxins will only make it a difficult cleanse and overburden the organs involved.
I read that if helps to overcome the urge to evacuate by breathing deeply. OF course if you just can't hold it don't force it. Have patience and it will get easier with each enema. Read a book and take your mind off of the enema itself.