Use this as a frame. It's an incredible frame that I learned from Mark Bell in one of his workout videos on YouTube.
When you encounter a difficult situation. Ask yourself this question. Are you stopping because it's hard or are you stopping because you can't do it?
Most of the time people stop because it's hard. It's still important to clarify if that's it because if isn't because you can't do it. Then you need to ask for help.
Ex. You're benching 225lb. You've done 8 reps before and want to try to hit a new PR of 10 reps. By the time you get to 8 reps your hands are shaking and you're breathing harder. This is the time to ask yourself. Are you going to stop because it's hard or because you can't do it? Try and fail or try and succeed? Make it reasonable.