I agree, usually I would translate 'beeinflussen' as influence, but it does not sound right to me here, if you simply use it both times in this translation.
It has been asked for specifically in British English, so, as an English person, I wrote what I think seems the best way to express what is written in German, in a British style that befits an Aphorism.
But thank you for making me think again about using 'influence' - it enabled me to incorporate it
I think there should be more parallelism between the first half and the second half of the quotation, as in the original.
For example, we could either use "control"
Don't waste your energy on things you can't control. Use it for things you can control. or "influence"
Don't waste your energy on things over which you have no influence. Use it for things you can influence. But meaning-wise, the translation is fine, of course!
I wote for IanMegill2's 2nd translation"Don't waste your energy on things over which you have no influence. Use it for things you can influence."
Thanks a lot all of you for your endeavours!
:-)