I tried to contact C a couple of times since then - such as sending her a Christmas card or "friending" her on Facebook (which she accepted, but then a couple months later she was all of a sudden gone from my friends list). Nothing much has come out of it though.
We have one close mutual friend in common (there was a group of 4 of us that were inseperable for a long time, but she is no longer friends with another of the group) - she updates me on C from time to time. I remember her telling me once that C really valued our friendship before the falling out, but just didn't want to put in the effort to re-friend again.
I think about C from time to time, and I just found out that she had another baby - that's two that I haven't been able to celebrate with her. She wasn't a part of my wedding, and won't be around for my PhD defense or when I have children. It still pains me to think of all the things we have gone through that we haven't been able to share together over the past four years.
It makes me so sad that two people that were so close at one time can't figure out how to forgive and move on. I really miss her, but there is really nothing I can do at this point.