Every line here is the set phrase 그러려니 하다 — deciding that something is simply how it is and letting it stop bothering you: 그냥 그러려니 해 (just roll with it), 못 지켜도 그러려니 하는 (even when it isn't kept, we shrug). It is acceptance rather than approval, and it is what people reach for instead of complaining about something they cannot change.