Lexi, I do agree with Jenny on one point - it's really best (I think) if you deal honestly and openly with the situation. Since you and your ex are split, what's the worst that can happen if you tell her? I'm wondering - are there kids involved, and you're afraid you'd lose access to them? I could understand that, but if it's just fear of coming out to your ex, well, she's already your ex, isn't she?
Yesterday my wife and I were on a long (9 hours) drive, just by ourselves, so I took the opportunity to get a little deeper on the subject with her. We had already talked some about it, and she had been supportive, so I took a deep breath and told her that it's NOT about how much we love each other, it's NOT about sex, or spicing things up in the bedroom, and it's NOT about sexual orientation, it's just about wanting my body to be in alignment with who I know myself to be inside. So we then discussed it in greater detail, but still not nitty-gritty, but she does now know that one option on the table is transition. And it scares her. At one point, she said "I feel like someone just took my life and flushed it down the toilet". She was silent for hours.
Anyway, yes, it's scary, but overall, regardless of what happens, I do think it's best. And Jenny, the scenario I described is EXACTLY why we hide ourselves - that was very painful for me to watch, and I cannot comfort her, or help her with her pain now, I just have to wait while she works through it. I have only recently become strong enough to do that.
So... Lexi... Hugs to you, girl - you can BE strong, even if it doesn't feel that way.