How to tell my mum Im bisexual?

It's normal to be nervous. Big news is big news, good or bad. But you don't have to let it be a big deal. Have you ever done something scary that wasn't so bad once you actually did it, like jumping off a diving board? It's kind of like that. Pick a time when you know she isn't busy, and trap yourself by starting a conversation with "There's something I need to talk to you about." That way, you can't back out at the last second. Then take in a deep breath and just say it. "Mom, I'm bi." You don't have to prepare a long speech about how you came to this conclusion or lead into it by talking about something else. Just say it, and it'll be over with. I don't know your mother, but judging by what you've told, she'll probably be okay with it. And if you just can't bring yourself to say it, try writing it down on a piece of paper and giving it to her, or leaving it for her. Good luck. :)