I've been through a similar confidence crisis, so I feel for you!
My little bit of advice, which worked for me, is just to simply pretend that you are confident. Fool yourself that you are confident. You know, deep down that you have nerves, but never ever let them show - because your horse knows when you are tense, when you are worried, and that only makes them worried as well, which can often lead to things going wrong, which is what you really don't want.
Like others have said, don't forget to breathe, as that helps you to stop tensing up, and just say to yourself, 'I AM CONFIDENT' and believe it. Stupid as it sounds, if you don't act nervous, your horse won't have any reason to be nervous either, and then everything should be a lot calmer for the two of you.
It worked for me - not a nerve in sight any more!!
Good luck - you can do it!!
Ride it like you stole it!!