>I feel as if this is the primary reason religions are created, even if a rare few sont believe in afterlives.
Religions are "created" to know, love and serve God. The trouble is that people, being fallen, lost their way and started mistaking God for His effects, worshipping the creature rather than the creator.
The whole "le death cult" meme of religion needs to die. If absolutely nothing else they help to prepare us to "make the journey", they weren't "created" as a way to make sense of death or to explain it etc. It was always an immature understanding of what we could expect but never could really articulate until the Lord Himself showed us by His death and resurrection.
>Some atheists vocally welcome death with no fear and in the case of Hawking and Einstein, denounce the notion of afterlives as something for insecure people to cope with the inevitable.
They say that, but we can never really knows what goes on between the soul and God in those last moments. Final impenitence is one of the worst sins you can possibly commit, however.
>Tl;dr how do I not fear death under the assumption there is no heaven or hell?
We can't answer the question the way you would like us to because Heaven and Hell do exist. Therefore, we say love God, serve Him with your whole being, pray, receive the sacraments, repent of your sins, and spread the Gospel to others. Above all, know that you could die at any moment, and would then be called to give an account of your life before the Heavenly Judge. Make sure therefore that when that time comes you may be found in a state of grace, or at the very least show deep final penitence. That said, we should always be humble and never just assume that we will make it, because that in itself is a sin.
If however you want to live life as though Heaven and Hell don't exist, you can't but fear death because then this is all there is, and your materialism is all that you have.
>I feel like only then can I entertain the notions provided by religion.
Your logic is backwards. You view death in a realistic way once you become Christian, not vice versa.