There is a difference between "murmuring" and "complaining" - and the difference is fascinating and counter-intuitive, but extremely important. (Murmer: to mumble or express privately an expression of discontent. Complain: to tell of one's pains, ailments) We are commanded not to murmur, imo, specifically because such pronouncements are done in a way that breeds discontent. (Think of someone slipping secretively among a group making soft comments to stir up anger or dissent.) However, in a very real sense, part of our prayers is supposed to be focused on our pains and ailments - asking for help dealing with and/or overcoming them. If we don't "complain" in that sense, we aren't acknowledging the atonement by trying to get help with our problems.
Look at D&C 121 and 122. If Joseph Smith hadn't complained, he (and we) might not have gotten one of the most beautiful reassurances in all of recorded scripture. We are commanded to endure to the end (and to endure well), but I can't think of a single instance when we are commanded to suffer in silence - except when it is to abstain from a public display of suffering, intended merely to highlight our suffering for public adulation.The same is true of repentance. It is totally dependent on recognizing our weaknesses and mistakes and transgressions and sins and, literally, complaining to the Lord about them. Of course, we should not complain to someone who cannot alleviate our pain (like to a mechanic about our blood pressure or a reporter about an internal family matter), but we must complain to the proper person to see any problem fixed. It's the attitude (complaining in humility) and the focus (complaining to the right person) that are critical.