I'm with Umbran. You can't be friends right now. Kindly ask her to respect your need to distance yourself from her for a while. You'll have a hard time moving on if you stay in close touch with her.
I went through something similar at age 21 (we weren't engaged, but had been discussing marriage), and it was made more difficult that I was really good friends with her brother, who was part of my gaming group. It took me a while to go through the classic stages of grief for our relationship. When I got to the anger phase, I kept it private so as to not screw up my friendship with her brother.
About a year later I had moved on, met someone amazing, and got married. My ex-girlfriend had also since married someone (whom she had been seeing while we were going out, but I won't go into that). A year after that when my friend, her brother, got married, we of course met again at the wedding - hard to avoid the sister of the groom when you are in the wedding party. It was a little awkward at first, but eventually we found we were able to reestablish some of the comfort we had together. It helped that her husband proved to be a great person, and that I was no longer pining for her since I had met what really turned out to be the woman of my dreams.
Give it time. It will suck for a while, but you will get through it. Take the opportunity to spend more time with other friends. Don't be actively looking for the next woman for a while, but at the same time be open to it in case she happens to be sitting nearby and asks you for directions.