The heart is a lonely hunter

I’m currently reading the book, and I came across this quote:

"She put her head on her knees and tied knots in the strings of her tennis shoes. What would Portia say if she knew that always there had been one person after another? And every time it was like some part of her would bust in a hundred pieces. But she had always kept it to herself and no person had ever known."

I’m a bit confused about what the character is referring to. Is she talking about a specific person or something else? For those who’ve already read the book, what or who exactly is she referring to?