If your thoughts don’t serve you and are negative your life can easily become a result of your thoughts. Negative thoughts don't serve you any good. But if you choose to create space for good thoughts that serve your highest and best, then let your mind run wild.
Overthinking is good for the "brain cells" to keep on ticking; just keep away from these thoughts of - "if only", "what if", "should have", "get around to it one day", and just concentrate on now for the next day.
Don't waste your time over thinking; it is clearly not worth it; do what you want to all your life; and take one thing at a time and dont try to save the whole world.