I am grateful to Tim Keller for many helpful insights. This especially.

The gospel is "I am accepted through Christ, therefore I obey" while every other religion operates on the principle of "I obey, therefore I am accepted." Martin Luther's fundamental insight was that this latter principle, the principle of 'religion' is the deep default mode of the human heart. The heart continues to work in that way even after conversion to Christ. Though we recognise and embrace the principle of the gospel, our hearts will always be trying to return to the mode of self-salvation, which leads to spiritual deadness, pride and strife and frustration in ministry. End result - ineffectiveness.
For example, Christians can derive more of their joy and a sense of personal significance from the success of their ministries than from the fact they are loved by God in Christ. Why? Their hearts are still operating on the principle--"if I do and accomplish all these things--then I will be accepted." (cf. Harold Abrahams in Chariots of Fire- "I have 10 seconds to justify my existence.") In other words, on one level, we believe the gospel but on another level we don't believe.
So why do we over-work in ministry and burn out? Yes, we are not practising the Sabbath principle, but the deeper cause is unbelief in the gospel! Why are we so devastated by criticism? The person whose self-worth is mainly in his or her ministry performance will be devastated by criticism of the ministry record because that record is our very self and identity. The fundamental problem is unbelief in the gospel.