I think the whole point of The Last Five Years is to show that it's nobody's fault. Their relationship had 'this will self-destruct' written all over it from the beginning. Jamie was rebelling and wanted to find someone who was against what he was supposed to look for. Cathy wanted to have a perfect relationship come into her life, fully-formed. He wrote a story about her and became the very thing he warned about (#3): "some people can't find success with their art/some people never feel love in their heart/some people can't tell the two things apart". He thinks he loves her, but he's not marrying her for the right reasons. He knows that things are moving too fast, but he's very impulsive, so he goes with it. Cathy is a romantic, so she never questions anything. She thinks that she's ready for this, but she's not. Her feelings of jealousy are awakened before they ever get married.
Part of what's so great about this musical is that there are no clear answers.
My friend Annie said that the first time she heard it, she spent the entire first act thinking that Cathy was needy and suffocating Jamie, and that the breakup was her fault, and then spent the second act thinking that Jamie was causing her to be paranoid and his cheating drove her to these extremes. I, on the other hand, the first time I heard it I spent the whole first act blaming Jamie, as it seemed he left Cathy with very little warning and she had become such a broken person due to how much she had invested in the relationship, and the second act I blamed Cathy, as I saw how she had driven him away from her.
Now I know better than that. It's not about the blame. It was nobody's fault. It just wasn't meant to be, and it took them 5 years to figure that out.
I starred in a short film called
Magnetic Personality. Check it out!