I knew at the point when I had never been so calm and rational regarding something about which I felt so strongly. I was ready to say "I love you", I didn't need to say it (and for me, there is a difference). When I said it, I did so because I wanted to share my feelings. I didn't worry about whether he'd say it back because I was saying those three words to let him know how I felt inside; what he may or may not have said in return was immaterial.
ETA: My feelings were also very natural. I wasn't preoccupied with where we'd be at XYZ date nor did I feel the need to strain myself thinking about whether things were "working" or how they'd work. I knew (not thought) we were meant to be and were TRULY a good match.