Trying to "tell myself you don't mean a thing" is hoping that you can convince yourself that it was nothing in an attempt to move on from what happened to you and that what you are trying to do is forget about what went on.
For me the song describes what could happen if you make a silly mistake during a relationship and watching the other person move on in life without you there with them. The pain of being in love with them still and having to see them leave you causes a pain that can only be considered as a pain from loving someone- in this song the greatest pain of all is love. The song is about heartbreak and the greatest pain - love.