sounds like you're running too fast to me
dylan i know cos that's what i still often do
either run a bit slower, or deliberately alternate walking and running, a la HIIT (interval training)
you need to eventually develop a sense of how near your limit you are, and slow down as you approach that limit. If you're not near it, you need to speed up. This might take a while, I'm still working on it.
good luck
roadkill what kills you? heart rate and breathing just too fast to continue? legs tired? stitch?