It is great that you think of serenity that is within you and that you control. You are exactly right that it is something that you choose to give away or to not give away! Nobody else can take it from you.
I realize of course it can be very challenging to hold on to serenity when life buffets you
But it is a wonderful skill to practice daily, to build and nurture your serenity. One way to think of it is a glowing sphere within you. It's safe there.
It can also be challenging to maintain serenity when the body is stressed with not enough sleep or healthy food or gentle exercise, so it's great that you're nurturing yourself in those areas!