Some days find you way outside your comfort zone. That's the day you present at a brown-bag lunch... and you've never talked to more than 2 people at once. That's the day you find yourself on the phone with a client explaining a bug... and for the first time discovering that you can't properly describe everything because they don't know the architecture of the system. That's the day you volunteer for the performance task force... when you're aware that "performance" has something to do with speed but don't know much else.
Stepping outside your comfort zone is a great thing, and learning new things is fun. Those are stressful days, though. They're days when you will question whether you can do it. They're days when you will go home and feel like you have no ground to stand on.
Those are days when you need your certainty.
Most of us have things we do really well, and that we're confident in. Some of us are really good at expressing ourselves in writing. Some of us can find bugs based on the barest wisps of smoke in the system. Some of us are awesome at system design. Hold on to those things.
When you have a day in which you're way outside your comfort zone, try to find some time to do something you're confident in. Think of your knowledge as an anchor in your own personal sea. When you're lost at sea, if you can just touch your anchor, it'll help you keep floating (and with that I'll stop the analogy before it falls apart!).
Mixing the new knowledge you're acquiring with the old knowledge you have can help you learn the new stuff faster by staving off the feelings of being overwhelmed. It frees you to learn what you want or need to learn. Try new things, please. And when you're feeling lost, remember what you know, and learn the new knowledge off that solid base.