Confidence is a key to getting there, and to somehow will yourself to jump into the dark water or swim through jelly fish. Everyone has their own way of building it. The best form of confidence is to simply practice something for so long that you can stand there on Shakespeare beach and know you can do it. 

Mental grit and mojo are also very useful. How do you train this? Ask any Nija. If you don't know a Nija then try asking a Tough Mudder finisher, an Ironman or a Goruck Tough. I became furiously addicted to challenges in my 30's. This grew from belt to belt in Akido as did my confidence. With 40 I switched to triathlon and stayed with it for 11 years. I was never good at it, but I was always a proud finisher. Learning to swim the front crawl with 40 was very challenging, but I wanted to be a triathlete. With 50 I began to become more interested strength and endurance.

Having mental grit is one of the most important attributes to have with such a lofty goal as swiming a body of water in the cold an through jelly fish, but few things can give you the toughness you will need. Last summer I met a 20 time Ironman finisher. He has attempted the Channel 8 times but each time he was taken out of the water. It is not a sure recipe for success but it doesn't hurt to know how to suffer and be acquainted with reaching your limits and learning how to keep moving forward.

After the Channel whopped my ass in 2013 I signed up for some more suffering with new intensity with the intent of getting more mentally strong. I entered the relm of Goruck. Goruck makes tactical rucks and they put them to the test at Goruck Tough events. Your prize for finishing is a patch. You form a team and an ex special ops officer grinds you down to your core being and enables you to look at who you are and what your're made of.