One of the toughest principles of Stoicism for me is Amor Fati. This quote summarizes the idea:
"From the perspective provided by the cosmic viewpoint, we can learn to love what happens in our lives. The Stoics propose that we should love all events, even those that appear tragic from our human perspective. Why? Because to do otherwise is ‘rebellion’ according to Seneca. It is an act of rebellion against the cosmos because we are claiming more power than we truly have. If we act virtuously with the intent to have result ‘A’ happen, but ‘B’ happens instead, “crying, complaining, and moaning” about it indicates we think we have more power than we have. The truth is that neither ‘A’ nor ‘B’ are ‘up to us’; neither is under our complete control. Only our thoughts and intentions toward ‘A’ and ‘B’ are ‘up to us.’ Therefore, a Stoic can intend that ‘A’ occurs and engage in actions to bring about that result; however, they must do so with a ‘reserve clause’ that acknowledges fate may not cooperate. This attitude is summed up in the old saying, “Lord willing and the creek don’t rise.”
Fate (God's Will) is going to happen, whether we like it or not. We can smile in its face, and accept it, or we can rebel against it, and, in either case, it will happen. Is your loved one dying? Have you been given a death sentence? Have you lost everything? Of course, you live your life by trying to influence events to the best of your ability. But, whatever happens is beyond your ability to control. Whining, crying, rebelling, or any other act of denial is a useless act. What purpose does it serve? Sorry to be preachy today; but, this principle is essential to happiness. Struggle with it every day. But, realize that the struggle is worth it. 