"The Struggle is when you wonder why you started the company in the first place.
[...]
The Struggle is not failure, but it causes failure. Especially if you are weak. Always if you are weak."
So untrue. You are not weak if you quit ! And conversely, you are not strong if you don't quit.
Sometimes, you should quit, and it takes courage to do so: when you gave so much to your project that quitting breaks your heart.