I'm beginning to think R is just a fantasy I'm clinging onto, but my reality is that the A is a deal breaker.
And it could well be, but it might help you to give yourself time before making that decision - or rather, wait until the first antiversary has passed. That time will be very painful and charged for you, no matter whether you're together or not.
I can see why there are so many triggers. For him to have cheated during what should have been a unifying and loving and supportive time - your pregnancy - must be very painful. I can relate, my H wasn't faithful during my pregnancy either. Has your H acknowledged just how hard a pregnancy must be - really acknowledge what you must have been going through and that even amidst the joy, there are still difficulties physically and emotionally? If he can acknowledge that and try to put himself in your shoes, he might really begin to grasp the enormity of his choices.
What would make you feel safe around the antiversary? Would it help if your H wrote you a letter on or around the antiversary, reflecting on his choices and giving you extra love and support on that day? Think about what will make the day more bearable for you. The first antiversary is the hardest, so be gentle with yourself.