I'm really sorry to say this, but if you are dead set on transitioning, a relationship will be tough. You can tell her that you are planning on transitioning, but it's likely to make things difficult if you aren't already in a serious relationship. And if transitioning is neccessary for your happiness, you can't give it up to please someone else. I'm sure you have noticed all the married individuals on this site who once thought that was possible as well. If you really want a relationship, please tell her. Otherwise, you may have to settle for a friendship. There is nothing wrong with that. I'm sure you would like to have her in your life given that she is special to you. As someone who likes men, I've had to settle for friendships with guys I really liked. It's tough, but sometimes people are too awesome to throw away just because they don't share your romantic feelings.
Again, I'm sorry that you are in this situation. I really hope you find some decent solution to this all.