I think the way Adele sings this is from the depths of her heart where she has loved and lost a love not in the sense of death but moved on with life choices where you know if you stay in that small town things won't change, your world won't grow, opportunities won't come your way. In the throws of young love, where the most incredible love nothing else matters at the time and because of life and changes the they moved on. Time has given her the opportunity to heal and want to apologise to a past lover for her behaviour/decisions. On the outside is looking back at the situation being able to reflect and know things could have been done differently. Hindsight is a wonderful thing. Love is a wonderful thing. One has never known true love until that true love has been lost. I love the depth of the meaning of the song.