You Stop When it Hurts
Stopping when it hurts is not the way to go. While you may find an exercise excruciating, this is actually a good thing because it means your muscles are actually being challenged. Instead of stopping entirely, try slowing your pace until you’re able to build up your stamina and increase your productivity.
Be aware if you’re lifting to light or not exercising enough, as those are things that need to be addressed too.