I actually think the Serenity Prayer is pretty valuable and wish it were reclaimed in some fashion.

So, one option is to take one of the precursors, like Epictetus's:

Make the best use of what is in your power, and take the rest as it happens. Some things are up to us and some things are not up to us. Our opinions are up to us, and our impulses, desires, aversions-in short, whatever is our own doing. Our bodies are not up to us, nor are our possessions, our reputations, or our public offices, or, that is, whatever is not our own doing

But, obviously, this is not optimized for actually repeating it. The Mother Goose rhyme is optimized for repeating:

For every ailment under the sun There is a remedy, or there is none; If there be one, try to find it; If there be none, never mind it.

but seems less on point (it seems to help to specifically call out serenity, courage, and wisdom).

One could just modify the front:

May I have the serenity to accept what cannot be changed, the courage to change what can be changed, and the wisdom to know the one from the other

The Serenity Prayer was posted as a rationality quote here and discussed. A few possible modifications were suggested, including this one from Mass_Driver:

Friends, help me build the serenity to accept the things I cannot change; the courage to change the things I can; and the wisdom to continually update which is which based on the best available evidence.

I think the words are right. My problem with it is that I associate it with people who use it as an excuse to give up. They focus on the idea that they can't change much of anything, so have to accept it. If I were to change it, I would treat the idea that I can't change something as a claim requiring positive proof and being subject to regular challenge. (I'd also filter it for things about which I care and things about which I don't care. But that's kind of beside the point.)

Max L.