The main effect of this is to make bad things, like tiredness, seem more hopeless and depressing than they need to be. But there can also be practical consequences, if I act on the false belief that a situation is more long-term than it is. For example, if I'm feeling unusually tired even for me, I might not ask for help, because I don't recognise that it's unusual; I think I'm always that tired. Or I might fail to put effort into maintaining a flagging friendship because I think it's been flagging for longer than it has, and forget the good times.
(This is the first idea I had that inspired this series of posts on fallacies. I've been planning the post and wanting to make it for months but had no time to type it up. But there was a comment on Slate Star Codex recently that related to it, so I thought I should get on and post it while that comment was still fresh.)