Just because some random metric tells you the stock market is overpriced, doesn't mean we'll ever see today's prices again.
Let me say that again. Even if the Schiller PE drops back down to 16 over the coming years, that still doesn't mean we'll ever see prices as low as today ever again. Prices could still keep going up and up, while the Schiller PE drops back down to 16.
Stop trying to time the market. You can't. Just put it in and forget about it.