President Roosevelt’s statement makes it clear: We, alone, control our thoughts and actions. I would say that this means you should look at yourself for the troubles in your life, they could be caused by your own actions. If you kick yourself for these troubles you would not be able to sit.

Roosevelt is saying "You are responsible for most your own problems." By the way, the grammar in the quote is poor, although Roosevelt may have said it way. It is better said this way: "If you could kick the person responsible for most of your trouble in the pants..." Notice the difference? "...in the pants" should not split the phrase that qualifies "person". This may be even better: "If you could kick in the pants the person responsible for most of your trouble..." This change keeps the verb "kick" closer to the adverbial phrase "in the pants".

Not 'between your legs'. Your backside. There's an implicit 'that was' after 'pants' in the quote.

Thanks a lot~

It means you are the person responsible for your own troubles. You would kick yourself in the pants (the sensitive parts between your legs) and you would hurt so much that you couldn't sit down.