It's difficult to say without knowledge of your water composition. It is likely your well water has a high residual alkalinity, making it difficult to lower the pH. I would recommend having your water analyzed, building water from distilled water, or using a spring water with a known profile. However, somethings you could try if you want to stick with what you have are:
Using an acid to treat all of your brewing water. How much? I don't know. Maybe around a 1/2 a tsp then test the water. You could get it down to around 6.5, then mash and see how it goes.
Using 5.2 from Five Star may help.
Gypsum may help too, but I don't know how much.
Without knowing your mineral profile it will be trial and error.