Throttle very difficult to turn?

Hello ,

I unpacked my Prima 5 again because it's a lot of fun to zoom around with it and attract a lot of people's attention.

But the fun quickly disappeared for me because my hands were starting to hurt a lot, with blisters forming on the base of my fingers and on the palm of my hand.

The throttle grip is simply very difficult to turn.
How can I easily fix this?