Tbh i don't know how i feel about people being sad for Kate for not being able to stick with someone. It happens, but also one's greatest love story doesn't have to be the last relationship you get in (from staying with them until death). Sure it's sad she doesn't marry someone she loves very much that loves her back as much, but why does she deserves to be rewarded with it and not a better mental health and control over her life? I don't know if it makes sense. Maybe it's amatonormativity that i'm bothered about. I don't think her character would be happy from ending with a great love story besides Cy.
Honestly, she's just not wired for romantic love the way most people are. Too much time spent being a cynic, too many reasons not to take it fully seriously. And, like you said, Cy was her one great love story, because he's similarly weird about romantic love. She can't really find anyone that matches this kind of connection. So, while on occasions she feels inadequate for the way she is, she doesn't actually yearn for a married life, she just goes along with proposals because she likes the partners that propose to her, and then inevitably she either fucks up the relationship or they realize on their own that the relationship is completely unbalanced because she doesn't really love them back the way they'd like her to.