Our water pressure (cold, hot, shower, toilet, etc... ) has been decreasing significantly over the last week or so. We have a well and septic. What might be the problem?

You could very well have a leak in your well line in the ground. I have seen this happen before.

Is there a water filter in the main line that needs changed?

What's the pressure gage say at the main water line into the house?

See if that pressure gage holds pressure. Cycle the pump and see how it runs and builds up pressure and see how long it takes and get back.