Santana just got up and literally poured her heart out to her girlfriend- Brittany, her girlfriend-girlfriend- very similarly to when she poured her heart out during Landslide. Both times, the words she was singing were meant for Brittany alone, and everyone else was just “furniture” to her (but she knew they weren’t that to Brittany.) She could say things with music when words just weren’t enough. She could tell Brittany her “private” feelings.
But when the furniture speaks, when an outsider- like Rachel or Mr. Shue- intrudes on her feelings, her reaction is to close up, to deflect, to make her feelings seem not as big and important as they are- because if someone saw all those big important feelings- feelings for Brittany- and criticized them or tore them down? Well, that would kill her, because Brittany is everything to her.
Both times Santana sang a song to Brittany in front of the club, Brittany recognized how big and important it is; she cries during If I Can’t Have You, and when Santana snaps at Mr Shue and lies about what the song is really about? Well Brittany’s not disappointed. It doesn’t phase her. She KNOWS.
She knows how big and important Santana’s feelings for her are. She knows she’ll always be Santana’s girlfriend.
(In her heart, Brittany will always be Santana’s girlfriend.)