Surprisingly easy to do. I have a LUKS laptop drive that I doubt I will ever get back into. Went on vacation and that was it. All I could remember is it's strong, random, >15chars and started with a S.
Since it was a laptop boot PW I generated (ranpwd), wrote it down, memorized over a week or so and then lost the piece of paper. Wasn't too concerned because I "had it"... until I didn't.
Or did you have it in some kind of password store that you no longer have access to?