nobody sees essence who can't/ face limitation. (21-22)
Circe seems to say here, "if you can see truth, you know things come to an end; you know there are obstacles, walls, and roadblocks in the way." If you are able to see Truth (with a capital "T"), then you have to be able to face the music, as it were. At this moment, we also remember that Circe is immortal, and so she knows no limits to her own life. Because she is immortal, however, she sees mortals die all the time. She is constantly abandoned, perhaps. Maybe being immortal is not so cool.
If I wanted only to hold you/ I could hold you prisoner (22-23)
Circe can prevent loneliness and abandonment really easily. All she has to do is to throw down some sorcery to get her way. Why doesn't she thrown down her sorcery?