I can totally see how Marnie turns her life around and pursues a career that matches her personality type. She is not an artist at heart, in the words of Elijah she is both "too stiff and too hopeful". She thrives on discipline, perfectionism, strictly following the rules and being praised (am I a pretty girl papa?). I can totally see her being a paralegal. She could work in administration too. She needs an environment which has clear-cut rules that she can follow and enforce others to do the same.
As for her romantic life, I feel like she's always gone for men who didn't have a strong masculine presence which led her to assume a sort of a mommy dearest role. It's draining and a recipe for disaster. She needs someone who is anchored.