Curious: How many cards did you create? And how often do you test yourself? You really need to do it daily with few gaps for it to be very effective.

I would recommend not giving up but instead altering your approach. I tried spaced repetition multiple times in grad school and completely failed at it. I then tried again over 10 years later (and much older!) and it was really like attaining a super power. The success is even more remarkable because compared to my university days, I am much less actively using the material in the cards.

In my case, the difference between the two attempts was sticking to the daily schedule[1] and getting better at creating relevant cards.

And keep in mind: SR aids in recall, not in understanding. There have been times where I look at the cards on a certain topic and do very poorly (consistently). I then realize it's partly because some key understanding has been lost. I then need to do a proper study session (get the book out, read for understanding, perhaps do some problems). After that study session, I get good on the cards and stay good for a long time. In the last 4 years, I've needed to do this perhaps twice.

[1] Yes, I do stop from time to time, but those are the exceptions, not the rule.

100% this.

Once I realized that understand is different from recall, it made a world of difference.

I created cards as I was learning the topic. And I was doing it daily or twice daily.

I certainly wanted it to work, when I first learned about it I thought it was the answer. But I seemed to get worse and more stressed out because I wasn't learning anything.

I'm not too fussed about it not working for me. I don't think there is a one size style of learning.

This has been my experience as well (especially with Anki), though I've always felt like my ability to memorize information to be poor in general. In my experience, I find it easier to memorize things rather through brute force/frequent repetition. To use memorizing foreign language vocabulary as an example; repeating words to myself, listening, or writing them out for 30-60 minutes everyday or every few days (as opposed to spreading out fewer terms over, say, two weeks). The conjecture I have is that my brain commits to memory more heavily based on frequency/intensity during short intervals of time rather than long intervals of time, but it's just a guess.

Not sure if I'm just an outlier, but I think you're right that many learning heuristics aren't one-size-fits-all. Hope you can find something that's effective (and enjoyable) for you. :)