The new breakers are actually GFCI
When you turn the GFCI breaker off you don't feel the tingle? Then it
cannot be leaking current to ground, or the GFCI would trip. (You do test
your GFCIs don't you...)

Even before you said that I questioned your story. Lets say there is a nick
in the insulation. Why would current flow through the ground, through the
pool, through you, and then back into the ground; when simply going to
ground (avoiding the pool and you) is so much more direct? "Charging the
entire ground" would trip the breaker, even if the GFCI was defective.
I don't know what your problem is (and you should have called an electrician
long ago!) but it is not a nick in the well pump circuit, unless that wire
goes through the pool.