Broaden your scope of focus. Instead of looking at your character as a collection of push button abilities try thinking about them as part of an an organically functioning party of adventures that is itself an evolving part of the world. Many elements of 5e design encourage players to view their PC as the freaking Mary Sue level Star Main Character Chosen One who happens to have a
Scooby gang they are accept being near
Once you do that it becomes easy to see how things like backstory go from not particularly important fanfiction about the character you wish you had to a more important thing like "
why did I turn away from my cushy life to be an adventurer risking life and limb. w
What was it before & who might my character know that I as player don't".
As to knowing your character such... That character exists as part of a group and it takes time for things like group dynamics to gel together just as it takes time for that group to find it's place in the world. Past editions recognized that better and provided more room for relevant growth accommodating choices than 5e as PCs advanced
You can't do that during character creation because your character isn't yet part of the world and the party can't exist yet as a whole beyond a conceptual bootstrap paradox