Anxiety Support Group
Anxiety is a physical condition marked by intense and persistent feelings of distress, fear, angst or dread. General anxiety caused by routine day-to-day stresses usually passes quickly and is experienced by almost everyone at one time or another. However, such feelings that linger over time and are very difficult to cope with, and which lack a clear cause, may indicate...

I'm sorry you're having anxiety about your upcoming medical procedure. I know the feeling all too well. Although I have not had to have a colonoscopy, I've had several medical procedures for my health condition in the past year. What I have noticed with my anxiety, is I always make things bigger and worse than they actually are. I go in scared expecting the worst, but afterwards, I realize it really wasn't that bad. Anxiety tends to makes us blow things up larger than reality. My best advice, maybe easier said than done, but try to imagine that everything goes well. Imagine the procedure being done in a timely manner, with little to no discomfort, it will be over before you know it and you'll be walking out of there thinking "hey, that wasn't so bad", then imagine the doctor calling you telling you the results and that it's good news. In the event something were to be wrong, you can deal with it then, but in the mean time stressing over a future event will do you no good.
Good luck at your appointment, you're gonna do just fine. Be easy on your mind, it's not going to be as bad as you can imagine it to be. :)
P.S. I've asked friends they all say the worse part of the colonoscopy isn't the procedure itself, but the part beforehand when you take the medicine to clear out the gut...that part may be miserable, but at least you'll be in the comforts of home.