I think Ordinary World by Duran Duran would be a good song for Mamoru, this is how I interpret some of the lyrics.
Came in from a rainy Thursday on the avenue
Thought I heard you talking softly
I turned on the lights, the TV, and the radio
Still I can't escape the ghost of you
What has happened to it all
Crazy, some would say
Where is the life that I recognize
Gone away
Mamoru went back to his apartment from walking around on the streets of Tokyo on a rainy Thursday night, when he's back at his apartment he goes to bed, when he's sleeping he gets visions of Princess Serenity and he can hear her say something to him and then he wakes up in the middle of the night and turns on the lights and gets out of bed, he can't stop thinking about the vision he had and he can't fall back asleep so he turns on the TV and the radio to distract him but he keeps geting more and more visions of Princess Serenity and his past life as Prince Endymion every day that he can't escape from and that former life has gone away so he trys to find out what the visions mean. Some people would think Mamoru is crazy for thinking that there's any truth to those visions.
I'm not sure about the rest of the lyrics of the song, if you guys want to add anything to this feel free to.
Album version
Single version