It is very common for me to hear the following statement after a client completes an exercise to failure:
"Oh! I felt so weak in that exercise!"
"You're welcome." is my usual response.
Feeling weak as a kitten after completing an exercise is exactly what you want. It's how you're supposed to feel. It means you've recruited most or all of the fibers in that muscle and ensures that positive tissue adaptation will take place.
If after completeing a set of an exercise you feel strong and hearty, you haven't given it your all.
So give it your all, feel weak, and watch yourself grow strong!