Girls' ideas are wild and unconstrained, and sometimes they don't have to be so complicated. Since you are a couple, she probably wants to share or express her love for you. Of course, if you don't believe me, you can just ask her why she sent it to you.
Judging from the meaning of the lyrics of this song, the lyricist just hopes that everyone can bravely pursue true love and try to preserve that beautiful memory. So, your girlfriend probably hopes that two people can cherish this relationship together.