The first person you have to know is yourself.
If you don't know yourself how can you objectively pick anything for your self.
How can you choose the direction you want to go. You have got to know your capacities, abilities, desires, and assets, both materially and personally. You have to know how fast you are how quick you are how determined you are etc . .
If you know that you are a determined person that won't quit until you have accomplished what you set out to accomplish then finishing a task will not be a problem. But if you have a deficit in determination and start tasks that turn out taking more time than you estimated then you will leave in your wake many unfinished tasks which will be a source of embarrassment and/or depression.
I have observed this phenomenon many times over. Almost invarably a diagnosis of depression accompanied these people.
I think you get it.
Know your self.
Link:For a much more detailed explanation go here.