Yeah, I stopped drinking in 2005, have the occasional one now and then but you could probably count my annual total drinks on both hands.
I really struggle to remember which year was which for stuff. Like @CharlotteB says, I might remember a stand out act or huge name, but even that's not guaranteed. I also struggle to remember where I've seen acts, as in which festival. I don't do loads of others, usually one or two non-Glasto ones a year and I'll know I've seen an act but often can't remember where. Or it's a real effort and a real 'A-ha!@ moment when I get it.