Facesitting

my girlfriend loves to tie me up and sit with her pussy over my mouth and my nose up her ass. She will force me to lick her pussy and will only let me breathe when
She wants me to. When she does let me breathe, which is every 40 seconds or so she lifts her ass slightly, my mouth still sealed, so I have to sniff her ass as she tells me to lick harder. She lets me breathe for about 3 seconds before pulling my head into her ass and pussy. This carries on for ages until she has cummed a few times. Sometimes she won't let me breathe until she's cummed.