I think she could have forgiven the infidelity, but she's having a harder time forgiving that he changed forever how she thought her life was going to play out. I also think she forgave and overlooked a lot over the years in her marriage because she really believed she was married for life with the end goal being that they'd live on the farm in their old age with their kids and grandkids nearby. It's hard to let go of a dream, but slowly, she is. Matt had the advantage of moving on way before Amy knew that she'd have to. It's easy for him to be snide because he's getting everything he's wanted.