I was briefly an agnostic at one point when I was around 15 for no more than a few days. I started debating with an atheist online and they posed some questions that I found difficult to answer. I started doubting if there was a god or if Islam was real. I remember feeling EXTREMELY miserable during this time. The whole world seemed cold, dark, and empty. It ended when I watched a youtube video about an Islamic convert, who said that on his journey to Islam, although he initially wasn't sure there was a god, he prayed regardless, "Oh Allah, if you exist, guide me." I did the same, I prayed this despite not being sure if my prayers were being listened to, and then was filled with faith again.