Nice signature line you have there mate, as someone that suffers with Anxiety etc that makes good sense. Another one I read recently went something like this.
A Rocking Chair is like Anxiety, it might keep you busy for a while, but it only goes backwards and forwards, you don't actually get anywhere 
Better to have a life full of "oh well's" than one full of "what if's" 
Don't worry about things over which you have no control. Having viable options/plans put you in control whether that is a second business to run alongside window cleaning, additional skills or qualifications that might help you if things go pear shape. Anxiety then diminishes when you are not worrying about the ifs, buts & maybe's.
Keeping in the theme of the post it would be a skill that complements my existing business if I was wanting to expand or find more lucrative work. i.e. abseiling or setting up a specific marketing strategy to target a particular market which is what I suspect the OP is asking about.
Personally I will not be doing any of the above which isn't an option so I won't be voting!
