My money is on a hole in your drop pipe, or a crack, or a failed joint in the string. That would explain never shutting off, only building to a limited pressure, as well an air bubble when you turn it back on manually. Could also be the tank, but more likely a hole in your pipe. Good luck.
Be prepared for the pump guys to suggest a new pump. When they run for weeks-on-end, they can overheat (without proper flow of water over the motor), and drastically shorten the life of the motor. A modern submersible is not likely to last more than 20 years under optimal usage (what's happened with yours is not optimal). Most modern tanks will last 10-15 years...