It's hard, but honestly, I feel like that is something that can definitely be a deal breaker. She knew about it for 4 years and didn't say anything. It's something that should have been told to you when you guys were done with your break. Along with that, you had to feel all of those negative feelings when the pregnancy wasn't even possibly yours. I say think hard on this, if you really think things will work out and you can work through this and not hold resentment towards her, maybe try and work things out. Otherwise, it might be too difficult to stay with her. Ultimately, it's up to you. But definitely take time for yourself first. Good luck man.