Jonah,
In general, I'd expect my spare percentage to be lower in competition than in practice, all we can aim for is to get them as close as possible.
It certainly sounds like you do enough practise that you should be confident of hitting single pins. You say that when you get up to shoot the spare in competition you feel like you're going to miss it, I assume you don't feel this way in practise? I presume one of the reasons that you don't experience this in your practise sessions is because you don't view it as important whether you miss or not (there is no game that can be lost as a result of not making the spare).
I would suggest trying to recreate the feelings that you get in competition, in your spare shooting practise sessions. There is presently a manifest difference between your demeanour in these two scenarios. The other thing you could try is not to think about the spare at all. In order to make the spare you have to do several things physically before you let go of the ball, try thinking about these (hand/wrist position, where your feet are going, where your target is) this may help to take your mind off the negative thoughts which are harming your spare shooting.
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Reporting from England
