I agree with those who believe it is both Jamie and Cathy's fault. Yes, he spent too much time focusing on his career and flirting with other women, while she was never really true to herself ("A Part of That"), but perhaps it was just time that they drifted apart. Had they been paying more attention to what was going on outside of their own respective life, maybe one of them would have noticed it slipping, but neither was willing to do that.
That said, after the first few listens, I was left with "that...(ahem) jerk!" feelings towards Jamie before I recognized that.
Call the understudy / I can't go on tonight / I'm drinking with my buddy / I'm getting good and tight / Before they raise the curtain I'll be higher than a kite / So call the understudy
I can't go on tonight