Hey seriousperson,
Like writeleft said (great screenname!), please talk to your doctor. There may be another way. My husband couldn't finish the prep, but they still did the procedure without any problem. It IS an important test. In Canada, they only do the stool sample test because they have nationalized health care, which isn't so great! The stool sample testing can't even compare to a colonoscopy in its effectiveness in spotting problems.
My husband was very anxious, too, and was able to take diazepam before his procedure. It's a very mild relaxer. He chilled right out and handled it just fine!
I hope you feel better soon and can talk to your doctor to figure out a way to get this screening test. You're probably fine, but if there is any problem in there, catching it early is the key to treating it.
Blessings,
Emily