Maybe by using the suppositories, you'll heal the rectum so you'll be able to hold the enema better. Also, have you tried just using half of the enema to see if you can hold that? My GI had me start out using half, and see if I could hold that, before "graduating" to a whole enema. Also, holding the bottle in your hands, or running it under hot water to warm it, can really help with retention.
Status: Remission since May 2009!
Symptoms began in November 2008, ~4 weeks after giving birth to my son
Diagnosed with pancolitis on 1/30/09
Meds: Apriso (4 0.375g pills ONCE!! daily), mesalamine enema twice weekly, Natural Factors Ultimate probiotic 12/12 Formula, OrthoNovum, multivitamin. Used prednisone (starting dose 40 mg) to get into remission.
Thread with links to UC resources and information: