In this context, I say serenity means to come to terms with... God grant me the ability to accept the things I cannot change, the courage to change the things I can, and the wisdom to know the difference.

I think it's a great, inspiring, and succinct prayer.

It's not an inborn thing - serenity is learned. It's calming, like a thought process to bring down anxiety. Kind of going outside to get a breath of fresh air, or simply taking a breather to think a situation through before acting or reacting on it.

I think therapy can help, but remember - all of this sense of peace and serenity is learned. It comes from within. Like stopping and smelling the roses type of thing. Therapy can help you learn what causes you to act the way you do, and to help change the way you see things to react and act in a healthier manner than reacting with anger, drinking, drugs, or whatnot.

As with anything, this stuff takes time, and it's hard to change patterns that seem ingrained in your psyche. But taking the time for yourself, learning about yourself, and learning to catch yourself from harming you is a good way to go!

ThesaurusLegend: Synonyms Related Words Antonyms
Noun 1. serenity - a disposition free from stress or emotion
tranquility, placidity, tranquillity, repose, quiet
calm, calmness, composure, equanimity - steadiness of mind under stress; "he accepted their problems with composure and she with equanimity"
ataraxia - peace of mind
2. serenity - the absence of mental stress or anxiety
ataraxis, peace of mind, peacefulness, repose, peace, heartsease
quietude, quietness, tranquillity, tranquility - a state of peace and quiet
Source:
http://www.thefreedictionary.com/serenity