Hm, I'm sorry to hear about the troubles in your relationship. I know you're not looking for advice, so I won't give any but I hope you can figure out a way to fix things d(^^)
Just for you, or for anyone else out there looking for an app that can record, there are voice activated recording apps available for the iPhone. I had one at one point and remember that I could set the duration of the recording time, and it was activated by sound. Maybe not the best fit for your situation, but the app I used could be set to be triggered by varying levels of volume, so if your discussions get loud, that might work for you.
Just look up "voice activated recording app" or "snore recording app" because a lot of them are geared towards recording yourself snore. Hope this helps.
Good luck.