What is/are in your opinion a/the Dream Theater songs that could represent a breakup? I have excluded Space Dye Vest because it seems the most obvious answer.
I would go with:
-My Last Farewell (the title says it all)
-Disappear (I know the song is about someone actually dying but it can also be interpreted as 'moving on' from a negative event in general)
-The Best of Times (remembering the best moments spent together)
-Behind the Veil (when the other one seems to not be affected by anything while you are desperately begging help)
-Endless Sacrifice (all the sacrifices you have done seems to not matter anymore)
-Far From Heaven (when you say to your loved one that she can't change your feelings even if she tries)
These were the ones that come to my mind.
BIG EDIT. Now the thread is referring not to 'songs' but actual 'lines'.