Ever wondered why Einstein would ever say that? I mean, he had every right to say that he was the only "fish" to be able to climb a tree and everyone would agree with him!
You see, Einstein in his early years as a student wasn't that brilliant as we thought he would be. He struggled badly under the Prussian Education. He also experienced a speech difficulty, making him the least charismatic speaker. He also failed the all important Swiss Federal Polytechnic entrance exams...
Yet despite all that "flaws", Einstein knew and understood his strengths. And he pursue them relentlessly, honing his gifts to perfection. And the rest is history - he became the founding father of Quantum Physics
SIMPLY because he identified his strengths and play them to the fullest.
Your first step is the same. You need to discover your strength, you need to redirect that wasted energy of complaining into something more productive and positive. Think about the situation that is making you feel so and look at it as an opportunity. Einstein did, he understood that there is always a lesson to be learned and a solution to the problem. If you think about the positives you have in your life rather than protesting and thinking negatively, you will focus on the positives that may come out of any situation. The more positive your thoughts are, the more positive your life will be.
So focus on the positive things in life and start by recognizing your strengths.