I like Christina's advice. When you are ready to tell her, I would be straightforward: "Mom, I've got something I've been wanting to share with you, but have been hesitant. I'm worried it might be a bit of a difficult subject for you or bring up some bad memories for you, but it's important to me to talk it over with you. I've been seeing a therapist for X amount of time, and have been diagnosed as bipolar. I've found a good medication that has helped me to manage it successfully for X amount of time now."

I just had this conversation with my own mother this past weekend. The hardest thing all weekend was finding 'the right time' to tell her. I finally realized it's not something that easily drops into conversation. So I said something like the above. It went well for us...hope it does for you, too.