What happens behind closed doors is often a shock to those on the outside of a relationship, and the unexpected bully is one of them. Many people believe all bullies are large people who use their physical strength to control their partner, but the opposite can be true. A smaller person has just as much potential to control another person, and they might do it without any physical threat. Most of them use their wits to threaten those they want to control, but it can still be a situation where a person feels threatened.
A relationship where bullying is present is unhealthy, but those who are on the receiving end of it are often concerned only with survival. They feel the need to give in to the person hurting them because they are afraid, and many of them know that others outside the relationship will not believe they are experiencing it. The lack of support in many cases is what keeps them in the relationship, but they are seldom committed to it.
A long term relationship where it was a factor often lasted a lifetime in previous decades, but modern people have learned that most partners who engage in this type of behavior are more afraid of the results than they are. Those who want their freedom will leave without a backward glance, but those who have not yet reached the end of their rope will stay in the hopes things will improve.
There is very little chance that anything will improve for the partner being victimized once they have given in to the bully, so their best bet is to leave immediately. If they remain, the likelihood of things getting worse for them will increase. They might not relish leaving everything they have worked for behind, but it could be a lifesaving measure that will give them freedom they never expected to experience when they do.