One of them I'm about a third of the way through, the other I have a prologue and what happens in the time jump between the prologue and first chapter.
Writing about yourself is probably always going to be the most difficult thing. I always have 2 projects so I can jump between them when things get difficult instead of getting frustrated and banging my head against a wall. Also, the first thing I'm working on is the only one with a deadline, the second is for fun and I'll decide if it'll see the light of publication later. This way, when I have to take a break, I'm still writing, still making progress, and giving my brain the break it needs without having any pressure to get it done. Then I can go back to what I'm supposed to be working on with a clear head and some new ideas.
So I'd say go ahead and write the fiction piece, especially if you don't have any deadlines on the memoir.