I'm guessing her husband is in a permanent coma or braindead. Technically still present in her life but at the same time he's not.
Or if we're going for wilder theories... Bella used to be extremely schizophrenic until she got therapy and acquired the proper medication. Medication she still takes to this day. While you might not think so, she's an extreme otaku (she's a librarian for a reason, she's got a a bunch of manga novels stored away). At some point, she convinced herself that her husbando was real and got herself married to him somehow, maybe in Vegas. They'll let you marry anything there.
After achieving a healthy mental state, her "husband" disappeared but her memories of her delusion remain. She hasn't completely let go of her manga husband yet because he still lingers in the back of her mind. And in truth, he's only one missed dose of medication away.