For some reason you can't get past what most would recognize as "your first love." which became "unrequited love"
We've all been there, done that, and can still remember the bittersweet feeling that it left on our soul.
It's a rite of passage from the adolescent mind to becoming a mature adult with the perspective of experience becoming wisdom.
The fleeting glimpse of a fairytale perfect love is always better seen from a point of looking back as the perfection of the girl who gave you your first kiss is only preserved in your mind because of the brief time that you spent together in a summer month when nature is omnipresently in a stage of rebirth and ones thoughts are of the opposite sex.
You can't go backwards but you can experience life to the fullest and create another deeper and more fulfilling relationship only if you allow yourself to live in the present and use the past only as a touchstone for your thoughts, but not for your dreams of the future.
It is said that there's someone for everyone out there and you need to free your thoughts so you can go forward in your life and can recognize the moment when she arrives.
So get out there and enjoy all that life has to offer before you realize that as we grow older and mature, we find that familiarity breeds contempt because the same traits that so endeared a person to us are the same traits that will now sound like nails on a chalkboard.