He never loved her as she loved him. That she was certain of, and watching him sing and play his guitar for her in the park hit the final nail on the head.
Terra had been walking to her school when she saw people running to the park saying something about Titans. So, being the curious geomancer she is, she followed the crowd to see what was happening.
Beast Boy was in the park with Raven and was singing her a song that he had wrote to finally ask her on a date. She was beat red, but her mouth was curved into the most beautiful smile and when she nodded to him to answer his question pure joy ran over his face. Beast Boy ran to her and lifted her up in a hug and spun her around laughing. And as he did, that was when he and Terra caught eyes. Terra was crying and hurt that he didn't wait for her and he just looked at her and mouthed 'I'm sorry, you were just a crush'. Unable to bear that sentiment, Terra ran from the happy new couple to her school vowing to forget about being a Titan and ever loving him. Though she knew she could not.
10 years into the future, watching the same scene replay in the park, but this time it was Garfield Logan proposing to Raven Roth, the heart wrenching pain and agony coursed through Terra again. She knew he never loved her and she was just a diversion until Raven returned his love and for some reason watching them glance her way and skip over her like she wasn't there that day, she allowed herself to finally forget everything and become Tara Marakov. Not Terra the Titan, in love with Beast Boy now known as Changeling. Sometimes the heart needs to fully shatter before it can be healed and you can start anew.