I'm not very good with 3D but, but I think you want to have velocity going forward which is
Vector3.FORWARD. The way your code is, right now, it looks like you're only using the y-axis (up/down) for both gravity and direction of movement. So, redefining your velocity variable at the beginning and maybe the moveandslide vector at the end should make it better.
The gravity is pushing down and the -roketHiziUP pushing up... surprised it moves at all... So, maybe using the Vector3 constants would make it more clear:
● FORWARD = Vector3( 0, 0, -1 )
Forward unit vector. Represents the local direction of forward, and the global direction of north.