“EVERY STUDENT MUST CHOOSE ONE OF TWO PAINS: THE PAIN OF SELF-DISCIPLINE OR THE PAIN OF REGRET OF NEGLECTED OPPORTUNITIES.”
Related quotes

Source: A Return to Love: Reflections on the Principles of "A Course in Miracles"

Source: Beauty for Ashes: Receiving Emotional Healing

“Given the choice between the experience of pain and nothing, I would choose pain.”
Source: The Wild Palms

“There's no regret more painful than the regret of things that never were.”
Ibid., p. 111
The Book of Disquiet
Original: Ah, não há saudades mais dolorosas do que as das coisas que nunca foram!
“Eternity and pain, pain and eternity — they are the only two things of which the universe is made.”
Source: War in Heaven (1998), p. 173

“It is a sweet, albeit most painful, feeling
To know we are regretted.”
The Improvisatrice (1824)