Throughout the first three seasons Princess Carolyn was trying to find the perfect man. She was burying herself in her work, because she couldn't find anyone suitable for her, or at least that's what she's been saying to herself. Turns out that in reality Princess Carolyn is unable to be in a relationship, because for her work always comes first.
She had five miscarriages, kept her old apartment, and lied to her fiancee about both. Yes, Ralph is a mouse, which complicates their relationship, but he really cares about Princess Carolyn, yet his every attempt to help makes her reject him even more. Just like BoJack, she also can't hold on to a good thing.
The emotional strain suffered during the fifth miscarriage seems too much for her and in order to spare her loved one the pain, she pushes him out of her house. She's been so strong and independent for so long, it's as if she doesn't believe a gentle and calm character like Ralph would be able to keep up with her.