It takes time to get to know and be comfortable with a therapist so it's understandable you feel weird about bringing up something big early on. I first brought up bipolar at 4 weeks with my therapist but that was 4 weeks that I had gotten to know her a little so I was fairly sure she wouldn't react negatively or laugh at me (my biggest fear). I also was pretty positive I was right.

When I did bring it up it was almost a non-event. I asked, she said yes and that the psychiatrist needed to confirm. Granted the day I asked I had manically adopted 2 kittens with no time to take them to my apartment so I had brought them into my session, which was a bit of a tip. We talked about how I felt about it for 5 minutes and then she moved on to something else, probably the kittens running around her office in an uncontrolled manner. Over time we talked about how I felt about the diagnosis a lot more but she gave me time to let it sink in first.

With bipolar every time you tell someone you probably are going to fear judgment. Your treatment providers should be the people who you don't have to worry about that with. But it does take practice to not assume that everyone will judge you. But not everyone will; plenty of people will surprise you with how kindly they take that news.

If you need to take some time before bringing it up I think that's fine. Get used to the therapist and then discuss this when you are not in the earliest part of that relationship. As someone mentioned tracking your moods would be a good idea and just watch that you don't start to go to either extreme too much, at which point it is time to bring it up.

But be comfortable with what you are doing. Not 100% as that might not ever be reached, but partially.