rueful smile

• wistfulSadly longing for something
• bittersweetWistful yet warm
• melancholySubdued, lasting sadness
• regretfulFeeling sorrow over actions
• sadEmotionally low

apologetic · tender · resigned · nostalgic · pained · sheepish · grimace · half-smile · sigh · remorse · longing · ache · knowing · self-deprecating · ironic · gentle · tinged · sorrowful

• smilesgrin, beam, smirk, simper, warm smile
• guiltguilty, guilt trip, guilty conscience, remorseful, contrition