I think, in many ways, the hardest time of our life is the time we are living in. It's all a matter of perspective. When I was in college, I told myself that I wanted to go back to high school and relive it, knowing that things weren't nearly as stressful as I thought they were. Now, I look back on college and want to relive it, knowing that things weren't nearly as bad as I thought they were. I think that being an adult and having responsibilities is much harder than any other period of my life was; however, I'm sure later in life I will look back and realize that this wasn't as hard as I thought it was.
Therefore, naturally, I would say that my current stage in life is hardest. I'm in my earliest years of teaching and I am planning a wedding. It's an incredibly happy time, but also very stressful. I don't have the self-esteem issues I did in high school and I don't have the time management issues I did in college, but it's hard in different ways.