Re: Huge plothole in book and I'm completely devastated.

Professional editor here, 11 years' experience. Just saying that in case it helps sink this in.

In my career I've found that 90% of all plot holes can be solved by starting with one question. "Okay, it doesn't work -- but if it did work, then how?" 

I know it sounds silly, but it's actually a very useful construction. Phrase it as a hypothetical. Pretend for a moment that someone else had planned it, some genius author who had it all worked out, and you're trying to puzzle out how it is going to unfold. Chances are high that the solution is simpler than you're expecting, just because you're too close to the subject. Take a step back and relax. Drink it in like a fine wine (as a college professor of mine probably still says) and don't get bogged down in what you think were your mistakes.