这话的翻译太难了，没法直译出这种意思和意境。你看看这个，应该会对你有帮助的：http://bbs.e5zj.com/thread-8345-1-1.html

i think it means "the moonlight brings light into my heart but then it will sink" (when the morning sun rises)

This moon could illuminate my heart, why then does it shine on the stream.

you are wrong to write this saying in chinese. The correct is writen as "我本將心照明月， 奈何明月照溝渠。” This mean that "you are sincere towards anyone, why are they hypocritical towards you back?"

"This will be my tacit moon, but the moon shines on the ditch"