Add a load resistor across the piezo element. Piezo devices are charge sources, the lower the resistor value
across them the lower the voltage produced. Because its a charge source you only get current when the
force changes - you can't easily measure a steady load without integrating the current or voltage signal.
In theory you could use just the piezo element with a voltage sensor than can handle large voltages, in
which case it acts like a charge source across a capacitor, which gives a steady voltage out (ignoring
leakage). But the voltages aren't very practical and leakage is variable.
You could try adding a (low leakage) capacitor across the element too, which should just reduce the voltages involved.