I feel your pain :/ Remembering all the concerts you've been to is tough. Here's a couple of things that help:
Good luck out there!
- You can use the Past Concert Search Engine to search for concerts by band AND location. For example search "Modest Mouse Portland" to see only concerts in Portland that Modest Mouse played. Super handy.
- Ask your friends that you often went to shows with. The buddy system is always effective :)