It helps that they're all in different fandoms, so it's easy to keep them separate.

But mostly, like you I only used to write one stiry at a time. Then I decided to actually use Google docs. It means I can just click on what story I want and it's instantly there. No loading up of Word and finding the right file, which always used to cement the mind set that I was writing that one particular story that day.

I know it won't work for everyone, but for me it really does.