This search will show you many others who have had the same problem.
It doesn't sound like a pump problem. It sounds like your water level in the well is dropping the more water you use, and the lower the water level, the lower the pressure from your pump.
You can add a pressure switch to make it automatic again, and adding a Cycle Stop Valve will take the place of the CU301, while being much more dependable.
Here is how I would fix it.