Then you can have these kinds of thoughts:
"Maybe it's OK to end my practice earlier than I planned. I think I worked hard enough today."
"Why do I need to suffer so much?"
"Do I really need this?"
"Practice shouldn't be boring - it should be fun and easy."
"I can't play any longer - there are more interesting things to do."
"My recital is far away - I have plenty of time to learn the music."
"Maybe I could stop now and pick up where I left tomorrow?"
"Maybe tomorrow it will be easier to practice?"
Here's my favorite: "Yesterday I practiced too hard. I should take it easy now."
Did you notice that when you persevere, push yourself and continue to practice as planned, your feeling is much better afterwards than at times when you give up? And the harder you have to push yourself to stay on the organ bench, the better your feeling will be.
Practice like there's no tomorrow.