Complex sentence: Robert and Susanne's relationship is now under still further strain because Robert's unintentional killing of Tamara is causing him great mental anguish: he blames himself for it and can find no way to deal with the guilt.
Split into simpler sentences: Robert and Susanne's relationship is now under still further strain because Robert's unintentional killing of Tamara is causing him great mental anguish. He blames himself for it and can find no way to discuss or deal with the guilt.