For excellent guidance on knowing when to quit and when to forge on through difficulties, see Seth Godin's book The Dip. His central premise is that the most successful people are the biggest quitters - they have enough self-belief to quit an unproductive course of action quickly.

As to suffering, the Universe provides enough of it to give us plenty of opportunity to learn our wisdom, without us seeking it out. The wisdom I have learned over the years is that every moment is precious, and only I can decide, case by case, whether the rewards of sticking it out through difficulties is worth it. Am I staying true to my values day by day? The journey, the learning itself, often matters. And there is no perfect option - we course-correct as we go.