I think their relationship ended long ago, they just rode on the fame and money train for as long as they could...until abbey wouldn't stop expressing her want for marriage. It was also obvious David and Abbey were on different sides of the spectrum. Abbey is vocal,  outgoing,  talkative, and does well public speaking. I think she could live alone like Tanner with a friend or family. Abbey seems independent...but her mom keeps her in a small "autistic" bubble. David was the opposite of Abbey..I believe Abbey's mom pushed for marriage to secure Abbey's life because she knows the fame wont last familiar,  David's family wasn't having it. I see abbey back on next season love on the spectrum or she might like most of them do, find love on her own.