Both of my kids struggled with this. A few things that helped my kids.
Graph paper. I write the problem in ink and they do the problem in pencil so when erasing they do not erase the problem on accident.
If they get it wrong, I work with them to find the error, not make them do it on their own. So often it is something really simple, and as they work through it again they find it on their own.
Remind them that it just takes time and let them sit with me when they needed to, to help check their work as they went so a silly mistake does not mess up the entire problem. They both know how to do the work, but would get caught up in the problem and have small errors. Checking their work as they go allows them to do their work, but not get discouraged with the time it takes and helps keep them focused and finish.
Also reminding them that it is just for a time, long division does not last forever 🙂