The girl in that video is an extreme case, and you don't seem to fall in that category, as I doubt someone behaving like that would be asking for advice on here.
That said, unless (or until) you've got a significant chunk of experience with meth, i would STRONGLY advise not indulging on work days. Or just keeping it to a bare minimum. Resist conversation as much as possible, avoid bathroom breaks, be mindful of how fast you perform tasks, and one big one is knowing how you present yourself to different coworkers when not on meth and adjust accordingly. You'd be surprised how much people will notice little changes in your behavior that you overlook.
AVOID VIEWING YOURSELF FROM AN OUTSIDE PERSPECTIVE.
Once you get even a little suspicion that someone suspects you're on something, whether imagined or not, you will become locked into this hyper self-awareness that causes you to view your own actions as if under a microscope.
At that point it is almost impossible to break out of and it snowballs into a very visible panic attack.
As far as the relationship stuff, my name's Paul, and that's between y'all