I feel the same. I'd originally planned to start trying for a second child when my first was 18 months, but the time came and I just didn't feel quite ready. Now my son will be turning four in a few months and I'm only just ready to try. Every other day, I feel like I change my mind -- sometimes I'm so eager to see a positive test, some days I'm totally content with where I am in life and don't want to risk changing that.
I see it as a good thing, in a way. I know that I'll be happy with my son if a second child isn't in the cards. It takes some of the pressure and stress off TTC. If it happens, I'll be excited and welcome a second kid; it it doesn't happen, I'll continue to love my son and look for ways to make his life fulfilling and fun as a single child. Maybe just see flip-flopping as a sign that you can be flexible and adapt to whatever may happen in life!