Coming out as trans is a ritual. Find a good time (late Saturday morning is best, I think) and place (home), and tell her you have something difficult to say (don't use the word "important" or she'll think you're proposing). Sit down somewhere comfy. And then tell her the truth of who you are. This ritual of coming out is a way of formally changing the dynamics of your relationship -- specifically, you're asking someone to gender you differently than before.
Do not manipulate her, emotionally or otherwise, into having the reaction you desire. That is not the foundation for a healthy relationship. You have to be open and strong for whatever her authentic reaction is. She might want to end the relationship, put it on pause, take a time out -- or she could be very supportive and caring. She might question whether she wants to be a lesbian or bisexual for your sake. Regardless, support her in whatever reaction she has, just as you'd like her to support you.
One thing is not acceptable, and should signal that you need to end the relationship -- if she insists that you're not really what you say you are, if she can't respect gendering you correctly.
Most people who deeply care for us will want to help. And a lot of these relationships (most, even?) will not survive. But that's true of all relationships, really.
Good luck, and know that so many people here will be here for you.
