If you want to be happy, be.
Well, it might have been easy for Tolstoy but I found it extremely difficult and at times even impossible. One cannot be happy just because one wants to be. Happiness is a state which is arrived at by a delicate balance of the internal psychology and the external world. One doesn't make oneself happy, but rather finds oneself happy.
In fact, not just happiness, but it is difficult to change any psychological state at will. Some amount of control is possible but that takes long term psychological training. For example, suppose I find myself being jealous. I know I am being jealous and I am simultaneously aware that it is not a right thing to do, but I am just unable to make myself not feel this emotion. And the consciousness of this inability makes me even more miserable. I understand myself and at times I want to change, but I cannot. I just had to learn to accept the way I am, along with my faults.