Santana had been sitting on her bed as she was remembering her encounter with Brittany on the beach. A whole day of events just seemed like a blimp in her memory, but it was the most significant out of all her memories. Now she has this letter explaining all of her feelings, one that Brittany wrote. That's what Santana wanted all along, for someone to understand how she felt. She never thought it come to her.
She had never in her life felt like she did now. She felt like Brittany was embracing her burning heart in dark warm ocean while it had been sitting with a burning hunger that was imploding on itself inside her. Now she was starting to feel an embodying love, a liquid warmth spreading throughout herself, that she had never had before Brittany was in her life again. 'I shouldn't be too surprised. She always had me.' Santana knew that she would never need Brittany but wanted her and the feeling that Brittany gave her.
The letter was a tangible piece of Brittany that she had given to Santana selflessly. Santana already knew that she had unknowingly, to both herself and Brittany, given her heart to Brittany that first night when she invited them all to stay with her for the rest of their trip.