Not FTM here, but I can really relate, I agree with much that has been said but, I have two things to add.
The first is that, my Stepfather, despite being the virile, fit former marine corps hand to hand combat instructor; had an exceedingly low sperm count and not only couldn't get my mother pregnant, but lost two other long term relationships over the issue. This bothered the hell out of him, it took him a long time to accept it, because it crushed his confidence and he was desperately looking for someone, anyone to blame for this...My mother didn't care, she already had 4 children, 2 of which she gave up early and 2 which she had a terrible time raising (and gave up after her marriage to my stepfather ended). The thing is, it took my mother literally years to convince him, that she didn't care. He let it consume his thoughts, by running through it over and over, and I could see it really took a toll on him.
My point is, you cannot (at least with current medical science) naturally have children; but you cannot let that define you. I have seen the terrible consequences that can happen when you take something like this, and tie it directly to your manhood. In my stepfather's case he brooded over the situation and ran it over and over in his head, ironically the Marine Corps says to avoid this very thing as it has been shown that running over traumatic events over and over in your head can actually lead to PTSD. Ultimately, your struggle with these feelings doesn't really seem all that different in substance than my Stepfather's but hopefully you can come to a healthy way of dealing with it, and not make the same mistakes he did.
My second comment is about a circumstance I was in, and perhaps it relates: back in 2011 my wife cheated on me. She had found some guy over the internet who was into the same fetish as her, and told her he would leave his wife for her, and they were having cyber sex sessions in front of the camera and talking of buying plane tickets and whatnot. I thought, since we were married that I needed to fight to get her back.
The thing is, I did fight for her, and I won, in that she came back. Despite me being trans (didn't seem to matter all that much to her) she came back to me. In retrospect though, it was a terrible mistake because all it did was buy me a few more years before she cheated again and left again. If someone is shopping to replace you, it seems to me like you ought to let them, because I don't think they are going to stop. Everyone is different, and I do hope for the best for you, but I would be very worried that if she is going to go so far as to cheat, that she might just not be worth it even if you can get through the current crisis.