all good things must (come to an) end
It is inevitable that good experiences will eventually end. I know you're sad to graduate, but all good things must come to an end. Knowing that all good things must end doesn't make it any easier to go home after a vacation!
All good things must (come to an) end.
Prov. All experiences, even pleasant ones, eventually end. It's time to leave the party, honey. All good things must end. We've had a lovely visit, but all good things must come to an end.