I think if you spin it right, it could be okay. However, you want to be careful. By saying it's a difficult decision, it can make you look like you were/are unsure of your decision. You certainly want to avoid that or at the very least go out of your way to make sure the committee knows you're %100 sure NOW! I would probably avoid it out of caution, and try to find something that would be similar to the difficult decisions that doctors must face. I think it's always best to answer every essay in terms of "why I'll be a great doctor," whenever possible.