Here are two suggestions. First, there's the 1992 song, "Sometimes Love Just Ain't Enough" by Patty Smyth (featuring Don Henley) https://www.youtube.com/watch?v=qdzbjUWu2VU. Second, Celine Dion's 1992 song, "I Love You, Goodbye" may also be fitting https://www.youtube.com/watch?v=UwESpbE0KuY.
More importantly, however, be careful of two things:
1) sending him a song with lyrics that express messages he may interpret differently than how you mean them. Be sure he gets the message that the relationship is OVER and understands why you're sharing this particular song. Put that part in your own words, so he doesn't read into anything.
2) continuing to communicate with him regularly probably prolongs the breakup and pain for you both. It's easier to do a clean break, although it hurts like hell for the first few weeks.