Where is the light at the end of this tunnel?

I swear things just keep getting worse and worse. Mark and I are fighting all the time... I just keep telling him that our marriage is over, and the worst part is that I really mean it when I'm saying it. I don't know what is going on; we were absolutely best friends before Matthew. Now I feel like we're just like every other married couple who can barely stand each other. We used to be each others' greatest support. Now we say such ugly things to each other that I can't remember what I liked or admired about him in the first place.

I don't know how things went so bad so quickly. And I don't know if things will ever be the same again.