Access iTunes for Windows Keychain
Various ideas:


Have you tried any typo variations on your iCloud password (since you mentioned you thought it was the same)? Another goofy thing about the backup encryption is that there's no second password field to ensure it's typed correctly.
If I remember right, Elcomsoft Phone Breaker's free trial will let you preview two characters from the backup password on Windows in case that will help trigger your memory. I'm not sure if it's compatible with iOS 10.2 and later backup encryption yet though. (I don't work on this program, but I work on other software that reads iPhone backups, and sometimes I suggest this to help people remember their password ;))
Did you setup that iPhone by restoring from a previous encrypted backup? If so, the backup password should be the same password for the backup you set up from.


Otherwise, you're pretty much left to trying to put yourself in the mindset of a few months ago to remember what you would have used for a password. :(