I have this habit, as well. I think that for someone who might have some insecurity issues and a family member or partner finishes their sentences, it is perceived as demeaning (despite the intentions of the one finishing the sentence). I have been accused by more than one ex of being a know-it-all, so this helped me realize that perhaps there was some truth to it.
I have been not listened to enough in my life that I have made an effort to become a better listener. Having an undergraduate degree in Communication Sciences, I do know that one should allow a stutterer to finish their sentences, and that has helped me be a better listener, too. Perhaps that is something you could focus on. There’s the saying that we have two ear and one mouth so we should listen twice as much as we speak.
I have a close friend who has other learning difficulties, and she does have a hard time with word-finding and she appreciates when I jump in with the word she’s looking for to help keep the conversation flowing. She likes that I ‘get’ her. When taking case histories from patients, the subtle differences between how they explain their symptoms can actually help us narrow down potential diagnoses, so I have to be careful to not ask leading questions or to jump to conclusions. It is very situation dependent.