Newbie question on bowing

[Enzo]

Hi there,

I am a newcomer to the instrument, and I am taking lessons. One of the things I find frustrating is the fact that the only time I can keep the bow absolutely parallel to the ground is when I look into a mirror. Else, I tend to "slide" on the string by an inch or so, depending on the length of the note. NBot so much on the E, A or D string, but the G. I use French bow and I play seated, using a 3/4 bass.
Is there a technique that can help?

Thank you