I was the last to know I was bipolar when I got diagnosed haha. I ended up telling a close friend after and she said she already knew that, and that exact scenario has happened more than once after. So, no, not super afraid of people finding out. I've been known to dabble with substances too, so I kinda prefer explaining things with a legal crazy than all the bs that comes with "ay, Sammy's on meth again! We can [too triggering to post] and she'll never tell because that'll mean ratting herself out!"

but I do understand how someone could have that fear, and I don't blame you at all for wanting to hide stuff from your roommate. If she has struggles too, though, that could be something you could put your toes in the water with and kinda feel out for yourself if you really do have to hide, and she'll probably feel more comfortable around you with mental health too if it all goes well. If she's been in and out of treatment, chances are she's not one of those "fix depression by mooning the sun" people, so you already have an advantage there.