Rest, see a doctor if your pain does not reduce.
Also errr, is not G502 a customizable mouse regarding its weight? Cannot you just open it and remove the weight inside or something?
If your pain is linked to carpal tunnel syndrome, you can:
-get a tablet to use your mouse less,
-get an ergonomic mouse,
-keep a regular mouse but hold it with the hand in a more neutral position, this is what I personally do. The idea is to tilt your hand so your wrist is in a more neutral position. Then, you click not with the bottom of your fingers but with the side. It sounds strange or dumb and it is not perfect but I know for a fact that it stopped the carpal pain for me.
But for you, rest, doctor and ergonomic mouse are probably the best.
Btw, to answer your question, I use a G402 but I often start working with my huion screen tablet.
Here is the trick I used to ease my pain: