It’s interesting to me that someone who is an adult (it seems) needs to convey a message of clear disbelief in G-d and is wondering if they should fast on Yom Kipper.
Please find a competent Rabbi to discuss the matter with further.
A real intelligent atheist (whom I have had the privilege of being friends with) will feel the need to study religion (question it and make fun of it), but maintain mutual respect towards religious folk.
You seem to be living a double-life. You go through the motions of Jewish culture, it’s not real!
Please take the time to study and learn: Aish.com or Simpletoremember.com