ANSWER:

This is why the prophet salla allahu alaihi wa sallam used to make this duaa the most in his life: O Allah turner of hearts, make my heart steadfast on your religion. 

One should be confident in his Iman and think positive in Allah while fearing leaving Islam or falling into sin. 

I had a friend who was way more pious than I am in many ways, but recently he became Athiest. Since then, I’m always worried about going astray and God Forbid, losing my faith. This gives me pain, anxiety and unbearable depression. I mean, what if I go astray? If he being pious can go astray, what if something of that sort happens with me? Please tell me what to do? I need help.