@fmsc,
My sympathies to you in this troubling emotional time. With some understanding and some frank discussion on both of your parts you can get past this impasse. - if you both are sincerely willing.
Perhaps the level of commitment is not capable for one or both of you? Have either of you been married or in a serious relationship before? How did that turn out?
That being said, something big is missing from getting an accurate view in this story. There should be something about counseling or a mediator or a discussion about counseling. You both need a perspective from a third party who is objective so you can say to each others there's a breakdown...and the hurt you both are suffering.
Once you go through that step, perhaps you can have forgiveness...but without more real candid discussion there'll be more hurt and suffering. No one deserves that.