Thomas Brooks in The Secret Key of Heaven argues for private prayer being not only very powerful but an absolute necessity.
"Private prayer is a work of absolute necessity, both to the bringing of the heart into a good frame, and to the keeping of the heart in a good frame. It is of absolute necessity, both for the discovery of sin, and for the preventing of sin,... and for the weakening of sin, and for the purging away of sin. It is of absolute necessity, both for the discovery of grace and for a full exercise of grace, and for an eminent increase of grace. It is of absolute necessity to arm us, both against inward and outward temptation, afflictions, and sufferings. It is of absolute necessity to fit us for all other duties and services,"
For Hudson Taylor, it was at times of extreme weakness and intense prayer that God guided toward bold new strategies for the mission and provided for the needs.
If the Brooks quote depresses you, remember that God gives grace to pray. Ask for it. In faith. Expecting sweet returns.