Not for Everyone
Countless millions, all over the world, have derived profound benefits from meditation, for over three thousand years. In the US alone, more than 27 million adults will have meditated in the previous year by 2017. Despite its rapidly increasing popularity since 1979, when Mindfulness-based Stress Reduction (MBSR) was introduced, meditation is not all things for all people.
Meditation practices and instructions arose at a time when prevailing worldviews, self-concepts, fears, anxieties and aspirations were fundamentally different from today. In his recent (2016) book, "The Meditator's Dilemma: An Innovative Approach to Overcoming Obstacles and Revitalizing Your Practice" Bill Morgan attempts to make meditation instructions more useful in our current context.
Many today come with unrealistic expectations of meditation: a temporary escape from stressful reality, floating off in a trance to a happy place for a brief length of time, then back to the rat race, essentially unchanged except for being a bit more rested & maybe a bit more efficient.
Most of us today are by nature, extroverted, action- & results-oriented, ie not self-reflective or introspective. We don't really know ourselves, and during meditation perhaps for the first time, may clearly realize that we don't want to or are not ready to know ourselves.
Furthermore, existing difficult body sensations or emotions can worsen, or present for the first time during meditation. Although this is a well-recognized, normal phase in any serious meditation practice, it can be an unexpected, unpleasant surprise.
“As you go deeper into your practice, there will be times of great inner tension followed by release to the point of weeping. If you have not experienced this at least several times, you have not yet really practiced.” Kornfield J, Breiter P. "A still forest pool. The insight meditation of Achaan Chah." Quest Books, 1985.
We may instinctively resist letting go of our psychological defenses and the memories / feelings / beliefs against which we defend ourselves. Although we're repeatedly advised to be non-judgmental, kind, gentle, & patient with ourselves during meditation, nevertheless, we may still need a mental-health professional to safely, slowly help release defenses and / or to help process the material against which we are defending.
The ultimate aim of meditation is "to discover a freedom that's independent of all circumstances & times" (Kornfield & Breiter above) and longer meditation retreats definitely enhance progress. However, meditation is a patient, life-long, immersive, transformative process not a dash to the finish line. And if we’re strongly armored, or have very painful burdens against which we're armoring, meditation retreats may be risky without the consent of a mental-health professional.
A top researcher re adverse experiences from meditation - Willoughby Britton PhD: http://www.theatlantic.com/health/archive/2014/06/the-dark-knight-of-the-souls/372766/
A thoughtful overview re adverse experiences from meditation by Matthew Brensilver PhD: