I'm offering my experience hoping that it might help you.
I became an atheist around the age of 12 or 13. I had been attending a mainstream Protestant church with our neighbors before that and chose to be baptized around the age of 11. About a year later, I decided I needed to starting taking this religion stuff seriously. Since we were supposed to believe what was in the Bible, I started reading it, from the beginning. I didn't even make it all the way through Genesis before I realized that I just simply couldn't believe any of what I was reading. And since I couldn't believe it, then I couldn't be a Christian, so I left peacefully. During high school I learned more of Christian history, which does not recommend that religion in the least. And early in college the "Jesus Freak" movement kicked in around 1970, which brought me in contact with fundamentalism, which strongly reinforced that I had made the right decision to leave Christianity. And then "creation science", which I started learning around 1981, was just icing on the cake.
So, as a young teenager right after I left the faith, I remember going through the thought process about morality and what to base it on. If I didn't believe in God, then it made no sense to use God as the basis. Fortunately, I had not been raised with an explicitly religious view of morality. I didn't learn until several years later that my father had left the faith because of the hypocrisy he saw; for his mother's sake, he continued to attend church until he turned 21. And while my mother was nominally Protestant, she didn't attend church.
So the moral training I was raised with was not overtly religious with arbitrary rules, but rather it was in the spirit of doing right by others, of courtesy, and of compassion. As a small child, I would open the door for others and continue to do so a half century later. Twice as an adolescent with fiercely bubbling hormones, married women to whom I was very much attracted basically offered themselves to me, but compassion for their husbands (one I knew, but the other I didn't) led me to decline their offers -- ironically, it turns out that my ex-wife had been cheating on me, so then so much for karma. In the discussion on this topic, some see fear as the main motivation for morality, whether that be fear of God's punishment of fear of the disapproval of others. Rather, I feel that compassion and empathy are much more important in morality.
Back to my thoughts while deciding what to base morality on. As a Boy Scout, I ended up choosing the Scout Oath and Scout Law. Yes, I was going through some eye-rolling at public religious pronouncements, but our troop leadership was based more on the Army model, so I didn't have too much of that to deal with. Later as a Scout leader myself, I learned that Lord Baden-Powell had pronounced the Scout Law to be superior to "thou shalt not" pronouncements (such as the Ten Commandments, though not explicitly named in that BSA magazine blurb) because "thou shalt" positive standards are superior to negatives. What I also did not learn until later is that Baden-Powell's Scout Law had only 7 points as opposed to BSA's 12 points and did not include a blatantly religious point, even though that religious point explicitly includes religious tolerance, which BSA does not practice (I was expelled from BSA for being an atheist, but only because BSA does not practice Scout Law nor its own published policies). But still, the positive values of Scout Oath and Scout Law were what I chose to aspire to.
In addition, I continued to think about morality. In 1985, I wrote down my thoughts, which I've posted here:
AN EVOLUTIONARY BASIS FOR MORALITY. The thesis was that morality is the social lubricant that enables us to not only live with each other but also to be able to function together as a society.
So that is what I am offering to you. I hope that it is helpful.
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