My sister and I had considered ourselves best friends since we were children. As young women, we shared an ambition for more independent lives, to know the world beyond small-town Texas, and we were united in our determination to succeed. We were inseparable. So it was shattering when, about 15 years ago, our friendship suddenly dissolved. It was as deep a sadness as I had ever felt, made all the more difficult because I had no idea what had caused the breach. Family ties held — brief conversations at family gatherings — but the intimacy of friendship, the shared secrets and holidays, slipped away. Attempts to repair the estrangement only seemed to make it worse. It took years to give a name to the emotional response I felt over the loss. I recognized it when my mother died some years later: grief. Just as I grieved the loss of my mother, I had grieved the loss of my sister’s friendship.

Loss is as much a part of human existence as breathing. It is an everyday event: a lost wallet, earring, investment opportunity. In most cases, we ponder what might have happened, get a little agitated, then quickly move on. But then there are losses that can’t just be shrugged off — voids that trigger a powerful kind of emotional response, like the one I had over my sister. Chances are, you’ve felt like this, too, if your home was somehow destroyed, you lost a job or a beloved pet, or your marriage ended in divorce. Maybe your health was devastated by a chronic illness or you experienced the death of a loved one.

Whenever a loss suddenly and irrevocably changes the course of your life, breaking the line from the past you cherished to the future you counted on, the complex feelings of pain you experience can all be classified as grief. “The basic core of grief,” says Holly Prigerson, an associate professor of psychiatry at Harvard Medical School, “is wanting what you can no longer have.” Yet grief is not a standard, one-size-fits-all response to life’s woes. Your reactions will probably differ with every loss you experience — sometimes unpredictably. (The death of a beloved pet, for example, might floor you more than the end of a marriage.) And how we each exhibit grief — emotionally, psychologically, physically — is as varied as our DNA. In fact, a new body of research (more than 4,000 studies on the psychology of loss have been published in the past two decades) overwhelmingly shows that there is no single, optimal way to grieve a loss, despite our ingrained expectations. Other findings are reassuring, too: The majority of us manage to heal, and many even find a positive outcome to our sadness. “Grief can be a bittersweet beauty,” says Robert A. Neimeyer, a professor of psychology at the University of Memphis. “It’s not something to be banished. It is a human experience to be lived, to be shared, and to be understood and used.”