I just told my parents, and they pretty much just said, "hm, alright". But with your situation, I would sit down and think of what to tell her.

As for it being "all in your head", and "You made yourself that way", that's not always the case. Some people are just born that way and drawn a sexual attraction to someone of the same gender. That being said, I think you should explain that to her. Also that there is nothing wrong with someone being bi or gay. Love is love, and there isn't anything you can do about it. Times have changed and she should be living in the 21st century. There isn't anything disgusting about it, and if she says so, ask her why she thinks that, and she should have a reasonable explanation behind it.

Also she is your mother you should love you for who you are. If you are bi, there isn't anything she can do about that, and she should accept that that's who you are. If she can't accept you for you who are, then it should be the same with her. People are who they are, and there is nothing you can do to change that.

Hope that that helped a little bit :3