It's hard to process all of the pain of the initial betrayal, and to feel like I am the only one feeling the pain now. Their pain was in not being together and is gone now.
If they both truly care about you, then there is (was, probably still is) some pain revolving around hurting you as well. Also don't discount that Mary may be feeling pain at the loss of your friendship as well (whether or not she hides it or is willing to admit it - losing a "best friend" is a blow).
Not to say that anyone of you has a greater or lesser pain ... but just that I don't want you to discount what else might be going on. Their pain at being apart may be gone, but it may lead to a different pain of knowing that you still hurt because they're together.
All of these varieties of pain and guilt are ones that my H, my b/f, and I dealt with over time. Some of the pain is still there - although much lessened by time and communication.
Sometimes compartmentalizing - for the time being - is a good thing. As long as it doesn't become exclusionary over the long term. Giving people time to recover is sometimes necessary.
I wish there were an emoticon for a soft, slighly wry smile that didn't seem so "perky" as the bright yellow smiley face.
It's hard to express the emotions w/out the body language sometimes ... but at the root of it all is a "hang in there" from me. If you all care about each other, it will get better.