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.