Oh I've been there! Being with someone with narcissistic traits is an endless mindfuck. They set you on an emotional/validation rollercoaster that is difficult to step off, because the rewards are intermittent. And your emotions have been manipulated and used against you so tapping back into them can be unsettling.
You need to give yourself time to feel your feelings. And after 18 months there's probably a lot! Feelings that you weren't 'allowed' to feel, feelings that you were told you were crazy for feeling, the underlying fucking fear which comes from being in a relationship which you secretly know is eroding your sense of self but being unable to move on.
It sucks but it won't last forever. However, I found that the time to get over the relationship was longer than you'd think given the length/shittiness of it.
Maybe do some reading, if you haven't already, about narcissism. But don't go too crazy. At some point it'll be time to look at how you can guard yourself against people like him going forward.
Which is done by... boundaries, self esteem and a strong sense of self.
Easier said than done I know 
But, you've already shown amazing strength and self awareness in extracting yourself. Stay strong, distract yourself and talk to others about what happened to you. And maintain no-contact with the ex.