I often hear people moaning about having had a bad training session and not performing as well as they think they should. A lot of the time I think those people think it's because they are low level and aren't good at "getting" jiu-jitsu... as if they have some sort of problem understanding techniques. The simple fact is that at any level the same thing happens, everyone can have bad sessions.
I had one today, and it would be easy to reel off numerous reasons why or explain the things which I feel made me perform badly... but none of it matters. Fact is it's done and dusted, just gotta take the lessons I can from it and prepare for tomorrow's training.
So if you're feeling like it's pointless trying to get good at jiu-jitsu because you're a 4 stripe white belt and you just got tapped by someone without even 1 stripe, just remember that black belts have bad sessions too... but we've learnt that the only response is to keep going.
Use your disappoint to fuel your next session :-)