and the portrait show seems to have no faces in it at all, just paint (11)
Our speaker has been shown a new reality by love, one in which art has ceased to be art for him. Instead, it's become "just paint." In actual fact, all portraits—on one level of reality—are just paint. The skill of the artist, however, creates a new reality for the viewer. The trouble is, our speaker's found an even better reality with love, and so the skills of the artists are lost on him. That's a real shame. Or is it?
it seems they were all cheated of some marvelous experiencewhich is not going to go wasted on me which is why I am telling you about it (24-25)
In these concluding lines, the speaker delivers the heart of his message. Love has awoken him to the bliss of his current reality (i.e., being in love). He's not going to go around anymore, trying to escape his reality by admiring other versions of reality, namely art. Oh no. He's going to appreciate the reality of his existence instead, which to him is far more enjoyable than any artistic substitute… except for maybe "The Polish Rider." Maybe.