There is a great rule that I must tell you: When two people are wrestling, if one of them is lazy and moves about heavily, he will fall, even if he is the stronger one. 

The same is true with the war against the Yetzer Hara. You can’t win this war with laziness and heaviness, which are the product of sadness and a closed heart – a heart as hard as a stone. 

You only win the battle with zrizus, which comes from a pure heart that is alert, clear from all worldly worries and sadness.

~ Tanya (ch.26)