Another approach is, if you know your next rep is going to have to be a push, just stop the set, and make up the reps in another set. So if you were supposed to do 5,5,5, but could only do 5,5,3, well you just do a 4th set of 2, so it ends up 5,5,3,2.
If next time you again find yourself doing 5,5,3,2, then just add another set, even if only a single rep.
In this way you are always doing more than you did before. Which is what progressive resistance training is all about.