If you lose something and unwilling to go on then...
Shirley was in love with Lelouch, genuinely and truly. Though there will always be arguments about her role to Lelouch or even the story for that matter, I can sufficiently say she existed. On that, I think it would be fair to say she was one of the main causes of the end.
I'm not holding her responsible, that would be senseless and ignorant. It's just her death had an adverse effect on Lelouch, which added another weight to his diminishing mentality.