Why is it possible to try so hard and constantly pray and have faith and still fail?

That’s a good question.  Success and failure depend on a complex network of factors.  You mentioned three: effort, prayer, and faith.  These are definitely very important, but what about wisdom, knowledge, skill, and support for example?  

I may have the dream to climb Mount Everest, but effort, prayer, and faith aren’t going to get me there.  I’m going to need a whole lot of knowledge, skill, and support from a team.  However, that doesn’t even take into account the bigger question of whether climbing Mount Everest is the right dream for me to pursue.  Wisdom is knowing the right mountain and the right approach path.

But even beyond that, we have to acknowledge that there are simply many factors that are beyond our control.  For reasons that we can only guess, God doesn’t protect us from many of those: storms, disasters, drunk drivers, illness, the sins and mistakes of others, etc.  Sometimes failure comes from within, and we can rightly blame ourselves.  Other times, we just have to acknowledge that we had a few bad breaks, and it didn’t work out for us.  There doesn’t have to be “a reason for everything.”

I suspect that most of the time, when we fail after trying hard and praying hard, we are really missing wisdom and support from others.  Thinking that we can handle anything with just us and God sound all nice and holy, but it just doesn’t work in practice for most people.  There is something mysteriously powerful about humbly gathering with our brothers and sisters, confessing our failures, and honestly asking God to heal us together.  This is why Alcoholics Anonymous and other 12 step groups have spread around the world.  There is an aspect of God’s grace and healing that can only come to us through other people.  

Whatever it is that is frustrating you these days, here is my advice.  Get together with some good friends and get really honest with them.  Tell them exactly what you’re struggling with, and ask for their support and prayer.  You may find the truth of this ancient verse: “A person standing alone can be attacked and defeated, but two can stand back-to-back and conquer. Three are even better, for a triple-braided cord is not easily broken” (Ecclesiastes 4:12).

[This is a response to a question shared on a "Talk Back" card at our church.]