Have you also had a decline in traffic in the last 18 months? It seems like the entire independent web is getting strangled out.
Site traffic is indeed down in recent years. The largest decline was when Facebook introduced "boosting," and stopped showing our posts to 90%+ of our Facebook followers overnight. I despise advertising, so I was unwilling to cave to their demand to "boost" our posts into ads. I'd have been happy to pay a reasonable monthly fee to reach our audience there, but that option was never available.
That big dip in traffic came with a big dip in donations, and as a consequence I eventually had to move from a part-time day job to full-time. The sharp reduction in free my time led to a sharp reduction in original content on Damn Interesting, which further shrunk the pool of people willing to donate.
This is a spiral that some would classify as "death," if I were willing to let it die. But it settled into an equilibrium where it pays for itself, makes a modest profit, and remains rewarding. Frankly I'd probably still do it even if donations dried up entirely, the research and writing give me a sense of purpose that would be difficult to replace.
I used to hope that a wealthy benefactor would discover us, and decide to fully fund our project for a few years, giving us space to realize more of the project's potential. But such offers come with strings attached, and I don't have the stomach for most of those. Perhaps I am broken.
At least what you have built will endure. Even if your invested just enough to keep the lights on, you would still have a trove of fascinating content. It's something to be proud of.
Also, your traffic might not be counting those of us like myself who use an RSS feed (a la Feedly) - those links don't go to your site, they just go to, well, the link. =)
And true enough that RSS traffic is largely uncounted, but there are many other indications of reduced visitor count--server-side logs, comment count on original content, number of email subscribers, and that sort of thing.