The Serenity Prayer of Reinhold Niebuhr* was used from about 1934 (Serenity Prayer - God grant me the serenity to accept the things I cannot (beliefnet.com).  Although written in the style of a prayer, I do not think it's relevance is confined to believers.  The first few lines in particular, can, with some adaptation, apply to many situations:

'God grant me the serenity to accept the things I cannot change; the courage to change the things I can; and the wisdom to know the difference.'

I have found it offers me a useful form of psychological counselling, a useful continuation of that I was offered soon after my life - physical and mental - was ruined by a spinal injury. 

Posts from distressed readers about unrequited relationships are, sadly, all-too-common; posts from members searching for what they regard as an ideal but where imagination has slipped over the boundary into fantasy: some appear to have lost the serenity to accept the things they cannot change.

Whilst imagination can be a powerful addition to the five senses - sight, touch, smell, taste and sound - in achieving stimulation, fantasy is unlikely to be achievable.  Some posts reflect a search for an ideal partner.  They fantasise about partners they are not likely to find and appear to reject what is achievable.

Is fantasy always something to be avoided?  I believe that in some situations, it is likely to cause distress if we expect it to happen: if we keep our feet on the ground and remember this, perhaps it is harmless imagination rather than fantasy.  There are other situations where fantasy could provide harmless consolation - terminal illness or bereavement for example.  In drawing this distinction between imagination and fantasy, perhaps I am searching for universal applications where none exists.

Those with chronic conditions might find the section after the opening sentence useful:

'Living one day at a time; enjoying one moment at a time; accepting hardships as the pathway to peace; ....'.

I find it best not to think too much of the future but others might differ.

I feel sure that the Serenity Prayer will offer guidance to some readers but I am unclear about how many.  Do other readers have any observations to offer?

*Reinhold Niebuhr (1892–1971), American Reformed theologian, was a professor at Union Theological Seminary.  He wrote and spoke frequently about the influences between  religion, politics, and public policy, and has been influential on many US politicians.