Sometimes saying a joke isn't worth the effort...

Sometimes saying a joke isn't worth the effort... I've had a bad habit recently of starting a joke and after some time trying to get to the punchline, I just give up completely. I have no idea why, but my speech has been progressively getting worse the past few months and it is so much effort to finish up a sentence... To the point of being out of breath and physically exhausted after a block. :/ My jokes aren't that funny and by the time I've finished a single word, the feeling of the joke is lost and I have no desire to finish it. Does this happen to anyone else? I just shrug it off and say something like the joke wasn't worth that effort, but that sounds like it might be insulting or rude.