Oh, DJM! I so hate for you to feel bad! (And I'm so sorry you've had all these expenses!) I think the folks who said to give her the diamond anyway are right. After all, that money was already spent before your promise!
We just had a big financial setback, too-- I lost my part-time job, which means that my salary has been reduced by 1/3.

Our anniversary is coming up in a couple of weeks, and I picked out and DH bought my gift (a double strand of pearls) months ago. Yes, it's a pricier gift than I would expect or possibly even prefer, given our financial situation. OTOH, it's already paid for, and I will enjoy it for the rest of my life! NEXT year, we can have a modest celebration, if our situation requires it. And so can you. But this year is done, so we should both enjoy. I hope your wife sees it that way, too.