Mama, I can understand your worry and frustration! Even when everything is going absolutely perfectly, it's normal to feel like a 24-7 "feeding machine". Also a burping, diaper-changing, rocking, carrying machine that gets pooped and peed on more or less constantly. I vividly remember preparing dinner for me and my DH for the first time after our first daughter was born, and just as we sat down, WAAAAHHH! So I had to sit there, nursing, watching my DH eat while my dinner cooed down on the plate. That sucked!
That being said, what you describe sounds pretty normal. Five minute feedings certainly could be an indicator of oversupply, as can the baby "clamping down" on the nipple during feeding. But without some of the other indicators listed above, I don't know that you can reliably diagnose oversupply or fast letdown. As a baby step towards treating possible oversupply, you might want to try a more reclined nursing position. Use gravity to fight the speedy letdowns, and make the baby more comfortable.
Some other causes of fussiness at the breast are listed here:
http://www.kellymom.com/bf/concerns/...e-nursing.html. Let us know if anything looks familiar!
You are doing a fantastic job. If you can stick with it, it is going to get so much easier!