It is actually two similes because it first compares horror on earth to a flower and then it compares it to a sun. They are both similes because each use the word like. Sebold is purposely using two positive and common images to compare to horror.
The Alice Sebold novel, "Lovely Bones," is about a 14 year-old girl named Susie Salmon. Susie has been murdered and while in heaven can see her friends and family still living. She sees their struggles to solve her murder and sees their pain and sorrow. Susie follows the map and arrives at the field. She meets Flora Hernandez who tells her the others will be there soon.
"And as Flora twirled, other girls and women came through the field in all directions. Our heartache poured into one another like water from cup to cup. Each time I told my story, I lost a bit, the smallest drop of pain. It was that day that I knew I wanted to tell the story of my family; Because horror on Earth is real and it is every day. It is like a flower or like the sun; it cannot be contained."
The quotation on page 186 is a literary term described as a simile. A simile is used to compare one thing to another using the words "like" or "as." A metaphor also compares two things that are compared, but "like or "as" are not used. This quotation uses "like" so it is a simile.
Both simile and metaphor are types of literary devices used to compare one thing to another. The difference is whether the difference is explicitly stated, or is implied. In a simile, the difference is stated; a simile uses a word such as "like" or "as". Metaphors are not quite so obvious. Two things are compared, but in a way that does not directly state the comparison. Your example uses the word "like", and is therefore a simile.
Love is a rose--metaphor.
Love is like a rose--simile.
It is a simile. The reason is because two nouns are being compared using the words "is like". If it were a metaphor, it would simply say that horror IS a flower or IS a sun.
In the book "The Lovely Bones' The statement you addressed is a simile. The young girl who has been murdered is watching things happening on earth. She is in her own private heaven after having been murdered by a neighbor. She compares the horror on earth using the word "like" several times. Similies use the words "as if" or "like" to compare two things.
This would be a simile because it is using the word "like" to compare two things.