Take a less gruesome situation. When you get a puppy you need to teach it obedience. The earlier you start, the easier your life will be later. Even though it may be hard work to get the message across, it's nowhere near as difficult as if you start with an adult dog. Anxiety that starts small is much easier to overcome while it is still small. And if you keep it small, it stays easy to overcome. With some made up figures, here's what I mean:
If you make a dentist's appointment this Monday for next Monday, and you really don't like dental treatment then the blue line shows what happens to your anxiety level in the meantime. Anything over 80% and you'll probably cancel. If, on the other hand, you acknowledged from the outset that you were likely to become increasingly anxious over the week you would be able to decide to devote time to reducing the anxiety every time it gave a little flutter. You can usefully use two strategies side by side.
1. Visualising yourself arriving at the dentist's feeling calm and remaining calm, using the techniques I've described, practising feeling calm, in other words, and taking that practice into the waiting room and then on into the surgery when you actually go.
2. Not ignoring the part of your mind that says "I'm bound to worry. I hate the dentist's" and not arguing with it, because you'll lose. Instead, agree but put off the start of worrying; "Of course we'll worry about it but not today. It's a whole week away". And later, "Yes, I promise we'll worry, but let's leave it until the weekend. That's plenty soon enough." With a bit of luck there'll be something to distract you from worrying across the weekend, and then you've only got a bit of Monday to deal with. The green line illustrates how that can pan out.
One final thing here. Notice that nowhere is it actually suggested that you should relish the visit; simply that unpleasant as you may find it, you can do it.