Your airline account activity pages
Usually the best single source. Often goes back years.
Start here, because it’s the highest yield for the least effort. Most major airlines keep a record of your past flights inside your frequent flyer account, usually under a heading like “My trips”, “Flight activity”, “Recent activity”, or “Mileage statement”. It’s often buried a level or two below the main account page rather than linked from the dashboard.
Log into every airline you have an account with, even ones you flew twice a decade ago. How far back the record goes varies a lot by carrier, and some only show flights that earned miles, so a ticket booked through a partner or on someone else’s account may be missing.