Haha oh dear yes this seems to happen a lot with me as well.
I'll go and do a dressage test, come out thinking it was horrendous, felt awful, didn't have the horse's brain at all etc. Get my scores back and be in the mid 60's for it!! Watch the video, and test looks pretty neat and tidy, nothing spectacular but no where near as bad as it felt.
But my rule of thumb is - if you think it feels bad, then there's something not right. The horse is evading an aid and you have to work through it, even if it looks and scores well. After all, we ride to have fun, even if we're getting 70% in a dressage test you're not enjoying it if you think the horse feels pretty ordinary.