Unlike the two other "dark triad" personality types (psychopaths and Machiavellians), there's a lot of nuance and differentiation among narcissists.
Some narcissists are worse than others; some are simply annoying but tolerable while others' selfishness and self-importance manifest as outright cruelty toward the people around them.
If they are sincere about getting help, they will take charge of the process themselves and make sure it happens.
If they don't, then you need to accept that they are not going to get help.
If you have any chance of making this relationship work, you need to heal your end of the codependent system.
In order to stay and make it work for you, you need to stop trying to be responsible for your partner's feelings and focus on taking responsibility for your own feelings instead.
Engaging in any form of relationship with a narcissist can take its toll mentally, emotionally and sometimes physically.
That means developing a strong, loving inner adult who speaks up for you rather than gives yourself up.
The term narcissist is used more widely today than it was ten years ago.
This is true in all relationships but especially so when it comes to narcissists, who generally hate being controlled.
In addition, trying to control in both overt and covert ways is your participation in the dysfunctional system you are in.