Question about IT's ending

Tried looking this up, but I couldn't find any posts about it.

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When Mike asked Bill how he'd know if Bill's attempt to somehow make things right had worked, Bill replied, "You'll know," or something along those lines.

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I expected the ending of IT to be bittersweet, but I didn't expect it to lean more bitter than sweet. What exactly is the "Devil" that Bill defeats, as referenced in the title of the last chapter? And how would Mike know if it worked if they all ended up forgetting everything anyway? I assumed that if Bill succeeded, they would stop forgetting. I suppose it was wishful thinking on my part.