If you think that just because you are on a spiritual mission, your endeavor will necessarily be a successful one, you are even more stupid than Don Quixote.
Count Keyserling lost his estate and home in the Baltics during the Russian Revolution and had to teach the esoteric discipline of "Wisdom" in 1920 - 1930 Germany, a time when probably few were ready for it. Still, he made a living in a profession he didn't want, and even though he was profoundly miserable, he nevertheless understood that it was all part of a grand plan to get his voice heard. He certainly inspired me with his insights, some 80 years after his writing. Count Keyserling accepted his fate and made the best of it.
"Success" is not ours to claim, even when we are fulfilling GOD's mission. The ego dictates what is "successful" and what not, while our soul might have a very different notion what we need to go through. Consequently, our ego it is bound to be disappointed at times, even for us spiritual travelers. A "failure" may in fact be the seeds for the next successful endeavor, or, as was the case for Count Keyserling, may force us to do what we are born to do even though we would never have done it on our own.
A bit better, but still not guaranteed, when it comes to happiness. We don't need "success" to be happy. You probably all remember the Christmas movie, "It is a wonderful life" in which George Bailey found happiness even while facing bankruptcy. An angel (without wings, sounds familiar?) had shown him that his life had meaning. "Happiness" is at our disposal insofar as it is a choice against the ego and for love. We spiritual travelers are good at that. We can't have it all the time given life's eternal ups and downs, but making peace with our ego - or leaving it behind altogether - is the whole point of traveling the WAY, so we are bound to be happy more often than not.
Meaning is guaranteed to a spiritual traveler, I would claim. Every step along the WAY has meaning, and if we temporarily forget our spiritual purpose and passion, the pain of separation is simply too hard to bear for us so that we are immediately induced to reconnect. Travel the WAY and realize that happiness, meaning, and our idiosyncratic notion of "success" are all nearby.