Relieve, alleviate, lighten, assuage, mitigate, allay are comparable when they mean to make something tolerable or less grievous.
Though they are often used interchangeably, they are clearly distinguishable.
Relieve implies a lifting of enough of a burden to make it definitely endurable or temporarily forgotten.
Occasionally relieve, when used in the passive, implies a release from anxiety or fear.
Sometimes it suggests a break in monotony or in routine.
Alleviate stresses the temporary or partial nature of the relief and usually implies a contrast with cure and remedy .
Lighten implies reduction in the weight of what oppresses or depresses; it often connotes a cheering or refreshing influence.
Assuage suggests the moderation of violent emotion by influences that soften or mollify or sometimes sweeten.
Mitigate also suggests moderation in the force, violence, or intensity of something painful; it does not, as assuage does, imply something endured but something inflicting or likely to inflict pain.
Allay, though it seldom implies complete release from what distresses or disquiets, does suggest an effective calming or quieting.