Lyric discussion by CharmingMan 

Cover art for I Promised Myself lyrics by A*Teens

...of course this was a song written and sung well before this in 1990 by Nik Kamen (of Madonna and Levi fame). This was actually a good song that did well in eUROPE - except for the UK where his previous songs did not do down well. This osng was dance remixed in 2004, and covered by a few oter artists. The original 90 version is better in my view than the other covers.
The song is clear in the sense it is someone waiting for their lover, promising that they will. I assume however they are not returning and they know that - possibly because they have died.