To let yourself go means to stop caring about yourself and your
outward appearance/hygiene, usually because of depression and/or a
traumatic event. Typically, a person who has let themselves go will rarely if ever leave their house unless they absolutely have to, and as a result they will
balloon themselves up to quite an unhealthy weight. Their confused friend(s) will often try to get them to come out of their shell, usually with no success.
I know that
your mother was a very
important person in your life, but you can't just
let yourself go now that she's gone.
