[Keith]

In this sort of situation, Steve advised that you question the belief, and 'drill down' until you get to something that is of questionable truth. (Note: In Steve's case, this led to subjective reality, but I don't believe that's a necessary outcome).

You say that you get nervous before tennis matches. I'll use that as an example, but I'm sure your answers doing this will be different to mine:

Q: Why do you get nervous before tennis matches?
A: The other guy might beat me!
Q: That's objectively true. Why does it matter if the other guy beats you?
A: Because I've put a lot of effort in!
Q: That's objectively true. Why does that matter? ie. Why should that make you nervous?
A: Because that's my main thing. If I fail after putting in all that effort, then it means I'm a total loser.
Q: Is that objectively true?
A: Hmm. No - I guess there's the idea of 'failing forward' - that failing just shows me where my weaknesses are so I can improve them. Hmm. And even if I turn out not to be world best at the moment, I'm still pretty darn good and I can get better. Hey, it's just one match - there'll be others!

IMO, just trying to overlay new beliefs on top of the old ones is going to work poorly (if at all) if deep down, you still hold the old belief. If you think the old belief isn't serving you well, that means it's flawed in some way - dig in, find that flaw and undermine it.

Also, try to practice being in the Now. The thing that makes you nervous playing tennis is almost certainly located either in the future (eg. "Can I win this thing?") or in the past (eg. "this reminds me of the time I got demolished"). Just live the moment you're in. The rest isn't an issue until later. (Note: This doesn't mean you can't objectively plan for the future ; just don't let you mind dwell there).

Like Eckhart Tolle says; "in the Now there are no problems". In the Now there are situations to be dealt with but they are only problems for the future (at which point they'll be just situations you deal with in the moment). (Sorry, it's kind of a hard thing to put into words).

I found this (second) approach very helpful in delivering my first speech at Toastmasters (I hadn't discovered the first approach at the time). I took a deep breath, centred myself and just focussed on each moment of the speech that I was delivering (which I'd drilled 'till I knew it inside out, BTW). I was still scared but an awful lot less than I normally would have been ; and my mind was crystal clear the whole time.

Hopefully some of this will be of use to you...