Optimism includes the assumption I can do something to change this situation for the better. Defeatism or pessimism includes the assumption probably nothing I do will make any difference.
Of course, when you're optimistic, you are more willing to take action to change things for the better, which increases the likelihood that things will change for the better. That's how optimism becomes self-fulfilling.
Pessimism can also become a self-fulfilling prophesy. If there is some area of your life that you have decided you cannot improve, you will no longer even try, which makes it more likely it will stay the way it is (or get worse).
Optimism is not the same as thinking positively, and it is actually easier than thinking positively. But it has far more evidence from scientific experiments proving its effectiveness than positive thinking does.
Read more: Optimism and Optimism is Healthy