This happens more often than you would think. Unfortunately the adhesive is the only thing that holds those screens in place and they degrade over time. This happens quicker if you are in a warm environment often, a sauna, in direct sun, or if it's getting wet often. This can be repaired but due to the age your best option might be to replace it as the repair is likely to cost you more than it's worth.
From what you have described all you may need is the force touch sensor adhesive gasket (14.99) but finding a shop with a low service charge is the issue.
Good luck and hope this was helpful