from the 'he's lying' point of view: she let him get away with it all the time ('these things they'll go away, 'years make everything allright''), but now this is the last time he will have to lie cause he's going away from her ('one last tender lie and then i'm out of this place). 'Something I wasn't sure of but I was in the middle of' could be about him being in love with someone else, but i'm not sure.
from the 'he's lying' point of view: she let him get away with it all the time ('these things they'll go away, 'years make everything allright''), but now this is the last time he will have to lie cause he's going away from her ('one last tender lie and then i'm out of this place). 'Something I wasn't sure of but I was in the middle of' could be about him being in love with someone else, but i'm not sure.