She can't get over with her ex. But she have to accept that it's all past now. Maybe they broke up or he is dead. But one sure fact is that her heart still belongs to 'the best one'. All that she got at the moment is second best.
I love and like this song much and it's guite personal to me as too. I'v broke up with my bf, because 'i should find 'better(?)'. So section where she sings "How do I get better, Once I've had the best, You said there's tons of fishes in the water, So the waters I will test" touched me much! . "I think you should know"