How can I prevent an exterior GFCI outlet from tripping so often?
Do not connect the ground wire to a ground rod at the pole! This is against the code and is very dangerous. 

For underground wiring, you want to use UF-B cable. This is rated for direct burial or use in conduit. You should have used scehdule 80 for the conduit as it is thicker and stronger. Schedule 40 PVC conduit can break easier and may be what happened. Depending on how deep you buried it, the conduit could have broke and damaged the cable.

If you used standard romex cable underground, that could be causing a ground fault because the paper insulation in the cable can wick water. Although the paper is not touching the copper of the line and neutral in romex, if water is in the conduit, the cable could have deteriorated and could be shorting to ground slightly. 

A big clue is that everything was working for a while and then it started to trip. Look for what has changed since it started to trip. Did you drive something heavy over the area the conduit is buried? Was there recent flooding?

But to reiterate my first comment, DO NOT GROUND THE GFCI TO A SEPARATE GROUND ROD! The circuit breakers will not properly trip during a short circuit condition in this situation.