I felt exactly the same before my driving tests (I won't say how many times it took) but there's a couple of things I did and I passed with no minors or anything.
First, this sounds daft but don't publicise the fact you're taking your driving test. by that I mean don't tell anyone except your instructor. A big part of my test nerves came from the expectations of others who, for perfectly good reasons, were so confident I would pass that it increased the pressure. The fewer people know, the less this pressure can happen.
Second, as others have mentioned, it's a lot of mind over matter. That is to say, treat it as any other drive where you have the opportunity to show the examiner how well you can drive. Look at it that way and it'll go a lot smoother and you'll actually drive better.
Finally, this isn't something I tried but my instructor told me it has worked for others. Get some rescue remedy, it comes in a little bottle and is a great relaxant and reliever of anxiety.
Good luck!