Where you used to be, there is a hole in the world, which I find myself constantly walking around in the daytime, and falling in at night. I miss you like hell.

This quote is expressing how much the speaker misses the other person. It is saying that the other person leaves a gap in the speaker’s life that they are aware of all the time, even when it is light out, but the feeling is particularly strong at night. The speaker is missing the other person greatly.