My take: "Because our cognition is unreliable, we can easily lose sight of truths we started out knowing as we walk along tempting-but-wrong garden paths, especially when strong emotions are involved."
In other contexts this is sometimes known as "being so sharp you cut yourself."
That's a good moral, but to me Lewis's quote seems to be more simply interpreted as an exhortation against successful doubt. Our thinking is certainly unreliable, but compensating for that with a fixed intention to keep believing whatever we're currently obsessed with seems like exactly the wrong thing to do; it essentially enshrines motivated cognition as a virtue.
Take off every 'quote'! You know what you doing. For great insight. Move 'quote'.
And if you don't: