I'm so sorry you're in this situation, OP, and I do sympathise about the fertility issues - that's a hard one.
For me, there were lots of things that might have been big enough to leave - there was physical violence; there was a lot of pretty abusive behaviour; there was dishonesty around money; there was dishonesty around plans for children (we're both women; she insisted she should TTC first and then she'd support me, but after DD was born she was unsupportive of me TTC).
But the actual thing was quite small. She'd been telling me for so long that she was struggling because we had money worries (her debts) or health worries, or because she couldn't do the things she enjoyed for those reasons. Then, things improved suddenly - we were doing better financially; she'd gone on HRT and said she was feeling much better. She booked a series of outdoorsy holidays doing the stuff she'd always said she missed. And on one of those, we were discussing the future, and she explained that, of course, if I were to get pregnant, I shouldn't expect the same support from her that I'd given. She would want to continue going to her various groups and activities, of course. I don't get the impression she thought this was a very significant conversation, but for some reason it just really hit me. I was the main carer with my DD, and we had a patch when she was working very long hours: at that point, she was quite clear that the priority was her work, and I needed to care for the baby (while working myself), so she could work the way she wanted.
It just really hit me that it would never be my turn to be anything but the giver in the relationship, and everything else was excuses.
If it helps, we've now been apart for a few months, and though I was really frightened at the start, it has surprised me to find how financially things are better than I expected, and I have loved parenting DD on my own - I actually think DD has benefitted from it hugely too.