Hey! Things like this is tough. Before you tell her that you’re bi, I’d sit her down and ask her how she feels about the lgbt community. That way you can prepare your response. If she is a supporter, then it’s easy. If not then I’d do research on how others came out to their parents and helped them understand the community. It will give you an idea of certain things to say to your mom. I hope this helps.
Tell her that you are. She will love you for whoever you want to be. Make sure to let her know that it’s a serious topic and that you have been stressed about how to tell her. Let her know it’s something you care about