I thought it was ‘Summer Sisters’ but Judy Bloom but I’ve just reread it and it’s not. All I can remember is that there’s a relationship in it that’s formed in childhood and ends in adulthood when the woman kills herself, and it’s described as her jumping off something (a cliff or a building I think?) in a confident swan dive, just like she used to do in the hay loft or lake or something when they were children. I couldn’t even tell you if it’s a friend relationship or a romantic one or siblings.