Seems to me that the two of you opened your marriage so that you could have more of the "fun stuff" (sex, excitement, hot dates) with other people, but neglected to deliberately set aside time and space for still having fun stuff with each other. No wonder your sex life cooled down - your marriage became all about bills and laundry. Where were the dates with each other?
I also think there is a way to save it without either of you ending your other relationships. You both have apologies to make, and need to rebuild your trust in each other. It will take a huge amount of inner work, soul-searching, and deeply honest communication - but most of all, I think it demands that each of you examine your beliefs about what love is, what marriage is supposed to be, and what kinds of commitments you can make and would be happy with. There are so many shades of love, and so many ways to love someone. Love alone is not enough to make a relationship succeed and bring joy and satisfaction - there are so many other factors, like respect, caring, consideration, honesty, trust, and affection. But I also think that very successful relationships can happen even if the love shared between two people is not the storybook kind of passionate romance. I tend to think that the ideas society has taught us about the importance of being "in love" is nonsensical mumbo-jumbo. More and more in my own life, I am realizing that the questions shouldn't be "Do I love him, and does he love me?" but "Do we care about each other?" And then demonstrating that caring, which is really how lovingness gets manifested anyhow, in the long run.