Otherwise known as beating your head against a wall...
If you are under the misconception that you could someday fill the attention-cup of the narcissist it is time for a reality check. There is no filling their vast reservoir. You can labor intensively for the rest of your life only to find yourself being condemned by the narcissist for never doing anything for them. (I'm betting you've already heard the narcissist say something like this.) At some point they will push you to utter hopelessness if you have insisted on clinging to the hope that you could ever be enough for them. My advice? Give it up now. Don't wait to hit the dregs of despair. Take that as sage advice from the voice of experience...because that is what it is.
Are you one of those people with a savior-complex? Are you convinced that the narcissist's redemption lies in the vast repository of love and hope you keep your heart? Alas, you have my very deepest sympathies. I assure you, at some point you will find yourself crashed upon the rocks of reality. Reality is this: no one can save the narcissist. How many more years of your life are you willing to waste on a project that is doomed to utter failure? A saviour can only save someone who recognizes their need for salvation. The narcissist is above all that. You are his inferior. You are the one in need of reformation so you can better fulfill the narcissist's endless demands.
And, please, think about this: When you expend yourself to fulfill the demands of the narcissist it is exactly the same as granting every petulant wish of a four year old. Has any four year old become better behaved by this indulgence? Quite the opposite. Neither will the narcissist. By sticking with him you are making him worse.
Stop right here and let that sink all the way in.
By staying with the narcissist you are part of the problem, not the solution. The only hope for the narcissist, as slim as that hope is, is to find himself utterly alone. It is the only time the narcissist has to confront herself. Only a crisis will induce a narcissist to seek out help from a professional. Chances are very small that the narcissist will stick with therapy long enough to deal with their real issue which is their narcissism. Once their presenting problem (i.e. depression) is fixed, they flit off never to be seen by their therapist again. But that isn't your problem either. Life is too short to waste it on someone who can never reciprocate, who is incapable of entering into your feelings, who will suck the life out of you and then kick your corpse as you lay lifeless reviling you for dying on them. Give your regard, your love, your energies to someone who can actually appreciate it.