Problem with Forward vector movement.

[shukenmg]

I have a problem with moving a character around a small planet.

Here is a video https://www.youtube.com/watch?v=-8F2J4szT6s

And here is the movement code :

The problem is that the forward vector and right vector is always changing (in the video the red arrow is the forward vector and the green arrow is the right vector)

So this causes problems in gameplay.

Is there any way to fix this?

Thanks.