Think of an enema as a hygenic, rather than a medical measure.
If you bathe before retiring, try to incorporate a slow, mild enema into the routine. You mention using coffee, presumably to stimulate the liver. That's fine, but I wouldn't try to absorb a healthful substance through the colon until the toxic garbage has first been removed.
I'd suggest two measures to help you evacuate, First, you might try injecting a few ounces of mineral oil into the rectum a half-hour or so before taking the larger-volume enema from a conventional syringe bag. You can use an infants bulb syringe to inject the oil, or Fleet offers them in a squueeze-bottle (the box with the orange label).
Another possiblity would be to add a little mild soap to the water, but be sure to use one with a glycerine base (the bar will be slighlty transparent). Nutrogena is the best known, and expensive, but I've seen bargain brands offered for as little as one dollar. Some health food stores also stock glycerine soap, sometimes as a liquid.
Best of luck, and please let the regulars on the board know if you're successful.