Something it occurred to me it might be useful to tell others about that I haven't yet said anywhere:
The only donation I've really regretted making was one of the first significant donations I made: On May 23, 2017, I donated $3,181.00 to Against Malaria Foundation. It was my largest donation to date and my first donation after taking the GWWC pledge (in December 2016).
I primarily regretted and regret making this donation not because I later updated my view toward realizing/believing that I could have done more good by donating the money elsewhere (although that too is a genuine reason to feel regret about making a donation, and I have indeed since updated my view toward thinking other donation opportunities are better). Rather, I primarily regretted making the donation because six months after donating the money I learned that if I had saved that money and donated it instead on Giving Tuesday 2017, I could have gotten the money counter-factually matched by Facebook, thereby directing twice as much money toward the effective charity of my choice and doing almost twice as much good. (I say 'almost' as much good because I think a smaller but nontrivial amount of good would have been done by Facebook's money had it gone to other nonprofits instead). (I in fact donated $4,000 on Giving Tuesday 2017 and got it all matched. I got all my donations matched in 2018 and 2019 too, and probably most of my donations in 2020, though matches have yet to be announced by Facebook. Other mistakes around this will go in a separate comment sometime.)
Reflecting on this more: Since I think marginal donations to some organizations do more than twice as much good as donations to other organizations (including AMF) in expectation, there is a sense in which missing a counterfactual matching opportunity was not as significant of a mistake as giving to the wrong giving opportunity / cause area. Yet on the other hand, regardless of what giving opportunity my 2017 self or current self might think is best, it's pretty clear that my mistake is one that cut the impact I could definitely have had with the money almost in half. That is no small error, hence my clear feeling of regret, and my frequent mention of the post EAs Should Invest All Year, then Give only on Giving Tuesday.