You either have your hand/wrist/elbow in a wrong position or are on your computer so much that you developed RSI. I'd suggest fixing your posture first before taking a break (or at least heavily limit the amount of time) from your computer and phone.
I've had RSI in my elbow and wrist and it's not fun... Had to go to physiotherapy for months whilst heavily restricting myself from being on the computer, phone and also working out (as that also strains your forearms / wrists).
An ergomouse can help you out, but you ain't going to game comfortably with that. It's fine for simple tasks such as browsing, office work, etc.
