let oneself go

verb2

1. •
   To relax and enjoy oneself without restraint.

   • Fill your lungs up, and let it out slowly. Let it out slowly and relax as you exhale. […] Just let yourself go, (name), and relax. — 2010
2. •
   To cease to care about one's appearance, allowing oneself to become unkempt, overweight, etc.

   • To be frank, you are an appalling sight at this very moment; you have let yourself go — but I have known you look quite ravishing. — 2012