I wouldn't think of it as "secret" exactly; your health difficulties are not most people's business? Why would you want to tell someone else? If your thinking is so you will be more comfortable saying, "Oh, the bipolar illness made me do it" if you act out or make a mistake, I think that will backfire. If it is because people pester you when you say you have been ill or have a problem, if people whose business it is not are asking you inappropriate questions and it's easier to tell than not, I think those sorts of people are likely to continue being inappropriate and give you their ignorant, thoughtless opinions of your illness, etc. So, why do you want to tell someone? Thinking it might make things easier for you without considering who you are telling and your reasons for telling that particular person is likely to make things more painful for you instead of easier and more comfortable.