In my attempts to Lucid Dream over the past few weeks, I've had similar problems. I think your problem is that your still getting a little shocked or excited when you go into SP. I assume this because you mentioned that your breathing got intense. This happens to me as well, and once my breathing becomes rapid I usually come out of the SP.

What I have tried doing to prolong the SP is as soon as I recognize I am in SP I tell myself that it is a very normal experience. While I tell myself this I also have been trying to take slow breaths. Anyway, It has prolonged my last 2 experiences of SP to 10-15 seconds up from 2-3 seconds.