I wonder how much is too much?

I am finishing up another big project for my employer on which I spent 12-20 hours on my own time. I have never felt delighted about spending my own time .. sometimes it's made me feel quite bad. But in the past I felt that the end result was deeply satisfying and perhaps spending my own time on it was the price I had to pay in order to be given the responsibility. Now, however, I'm finding myself feeling a little bit used and I'm concerned about getting burned out. I find myself wondering if it's true that by working extra hours I marked myself as a patsy and ensured that I'd be placed in this position again and again and, now I wonder if there's any graceful way I can extract myself.