I've definitely failed in my sacrifices quite a few times so far, and I know I will again in the next 25 days. But I've found that if I keep this quote in mind, even in my times of failure and weakness, I'm much more willing (and able!) to keep in mind why I'm sacrificing and why we journey with Him these 40 days.
I have a tendency to let myself off the hook too easily this Lent - mentally I tell myself it's because I'm pregnant, but just because I'm growing a person inside me doesn't mean I shouldn't sacrifice just as well as every other year. I have to remind myself to "be stern" and not give myself too much slack for being pregnant.
We have only a little bit left in Lent, so keep heart if you're struggling as much as I am. He sees it all and appreciates even our failures in efforts to join His suffering.