It's a mental trick. But if you can let go of "having to catch as many fish as possible", and instead take it as a challenge to catch a few on small flies. Then the lack of confidence isn't an issue. It's more like you are on a mission to do something you think is difficult, rather than take the easy way out and go back to what you know.
Then once you start succeeding, confidence begins to come naturally with success.
At least that's how I mentally approach new things where I initially lack confidence, whether it's a new pattern or technique, new stream, big fish hunting, etc.
If you're a hunter, it's like bow vs. rifle. If the goal is to kill a deer, a long time rifle hunter sees archery as an inferior way to put deer on ground. But if you make the goal to kill a deer with a bow, the equation changes. And after pursuing that goal you discover a few inherent advantages.
