God, it is said, respects our power of choice. If you spend time in devotional exercises and enjoy them day-by-day, but still err within minutes or an hour after your time of spiritual devotion, something else must be taking place.
Could the experience of erring itself be a test? Could the test be whether you feel that the Christian life is not what it's purported to be? Does it make sense to spend time on your knees and in the hour of prayer and still err because, deep down inside, you want to err anyway? Sometimes you might not want to err, but slip up and then the challenge takes a different turn.
Do you let errors, faults, slip-ups--fill in the blanks--discourage you that your Christian experience is false or not worth the trouble? Or do you persevere because in spite of your errors, you still find so much beauty and sustenance from the time spent in prayer and bible study each day?
If you've enjoyed the Christian life for a while and find it to be a valid approach to the challenges of life, you ignore the fact that something is not quite right with your life and you don't understand what it all means, but, nevertheless, you don't cease being a Christian in spite of the failures. If anything, your failures indicate that you need Jesus more and more. You may not be the successful Christian you'd like to be right now--or ever--but it is better to be a struggling and imperfect Christian than not to be a Christian at all.