Crystal Line by Anne McCaffrey

When Killashandra Ree joined the mysterious Heptite Guild, she knew that she would be forever changed. Crystal singing brought ecstasy and pain, near-eternal life...and gradual loss of memory. What she hadn't counted on was the loneliness she felt when her heart still remembered what her mind had forgotten. Fortunately, someone still cared enough to try to salvage what was left of Killashandra's mind. But she would have to learn to open herself--to another person, and to all her unpleasant memories