I disagree with the other posters. I don't know the ages of the other posters, but I don't think it's important to emphasize whether you were married or not. In this day and age, people are often more open to the idea of having a committed relationship without the legal confines of marriage. No one but you and Zack know the extent of your feelings to each other and whatever commitments you made to each other through your actions or words. I wouldn't want to disrespect you and your relationship with Zack by ever implying that his walking out on you is not hurtful and that you face a tough decision. Your presence here on this board shows you have very deep feelings for him. Society needs to come up with another word other than "boyfriend" that applies to a guy you aren't just dating, but live and share a life with -- someone with whom you've planned a future and love as much as other people love their spouses. It's not up to society to judge why others do or don't get married.
Allow yourself and him time to cool down. Maybe with some space, you will gain some clarity on how to react.
If ultimately, given PLENTY of time to contemplate, days, weeks -- whatever it takes, you decide you can't do this anymore, don't hop into another relationship right away. The end of any relationship -- whether married or not -- is a huge change and you need time to come to terms with it and process your emotions. It can take months, if not one or two years, to be in a healthy emotional state to enter into another relationship.
If you decide that you guys can work it out, great and I wish you both the best.
Ultimately, that decision is up to you, but I know it is a tough one and I know how much what he has decided to do hurts - I've been there.
"When one door of happiness closes, another opens, but often we look so long at the closed door that we do not see the one that has been opened for us." - H. Keller
"Change & growth take place when a person has risked himself & dares to become involved w/ experimenting w/ his own life." - H. Otto