Iam finding it very difficult to get over my 15 years realationship with my ex who i believe a narcissist. he seems so happy with his new girlfriend?

Narcissistic Ex?: If your ex is narcissistic, his new girlfriend now has a problem on her hands. It can be hard to see him (even with flaws) with a new partner when you're not, yet. Your mind can play all kinds of games about it. Maybe he was distracting you from emptiness that you can now fill with healthier relationships? Best to busy yourself with positive things in life which are unrelated to him. Good luck!

Seek help: He might be happy with her, but if he's narcissistic than likely she might not be as happy. All that said, if it wasn't working for you when you were with him, why are you still thinking about him? Whether he is or is not happy, you need to move on and get a life for yourself. It sounds like you are stuck and need help moving on. I would suggest therapy to help.

Narcissist: He is now someone else's problem, and if he is a true narcissist, he likely will drive her away at some point. Find a therapist to speak about the recovery process after dealing with 15 years of it (like exploring why you stayed so long.) A narcissist is the way they are for reasons that have nothing to do with their loved ones. They lack the empathy to own their part in their own fate.

Please see a Clinical Psychologist with expertise in multiple types of psychotherapy to help you cope with the loss of your relationship and seeing the new relationship.