An anti-scalding device is what you want. It does mix the hot and cold, and when it detects any 'change' in the temperature you set it to, it will either add hot or cold water to the mix so you don't get burnt. They work very well. DO read about the maintenance on them. I have heard of them seizing-up especially in well water environments. If you have a filter in your system AND a softener, I'm pretty sure you should be OK for some years to come. When you go to buy one, do tell the plumbing store (department) that you have well water, and whether or not you have a filter & softener system.
My best to ya and hope this all helps. IF you need more info, post up!