Well, imo (and it is just that - an opinion - you can tell me to f*** off and I'm cool with it - lol), if you need to get that out of your brain, get it out, but keep it separate from your story for now. You should have all the backstory you want as a writer - but only for yourself. I say this because you won't know what is necessary to put into the story, and you don't want to bog it down too much with the past or have it distract the reader - you want to keep the pace going. So if you keep it separate, you'll get your thoughts out and can put in what you need to, or figure out a different way to add it in, but you will have it at your disposal.

Put what is relevant to the story in flashbacks or try adding it in as dialogue, a diary entry, talking to a headstone of a deceased loved one, or their living pet, a taxi driver, a baby, have them shout it from an edge of a canyon or a note in a bottle, express it in painting/artistic ways, self-destructive hobbies - like skydiving/base jumping, shark diving w/o a cage and covered in seal blood, they go rolling down a steep snowcovered hill - bc they can't deal with their past . Get creative writer - the sky is the limit!

They should be used to show a character's motive, fill in something important, or if you can't figure out another way to put it in.

Too much and a reader gets bored, and too little leaves the reader wondering or confused. Try to make a good balance, make sure it only makes the story better and moves it along, not slower or confusing. ehh, I hope that gives you a little help. Keep on going kid!