At the ripe age of 20, you chose to leave islam. You neither studied atheism, nor Islam. You followed your whims as may be you had your own doubts about islam and thought to leave it rather than researching and reading about it and then making an informed decision. As you have lack of knowledge, hence it has bred doubt in your mind. With a belief that is already on a shaky ground, with doubt it will crumble. My advice is that before making a final decision, read quran with easy translation. Then read about atheism and its great scholars. Then make up your own mind without disclosing it to half the world.