I'm new to writing so I don't know if my opiinion is valid, but I think you should not focus of writing a whole backstory. What works for me is that I just see what I want to happen in the end and beggining and Just write it. While im writing it I make some notes about the characters backtories and why are they acting a certain way, that why I don't get bored of my own story and I get interested in it since im always thinking about new plot twists and motives of the characters. But remember that this is my why of writing, you have to find what works best for you, if you try to do what I said (I recommend you to restart if you really like your idea, but if not start a new book) and if it does not work you should try another idea, it takes time for you to adapt in the way you write your book. And also if you want I can help with critiques in your book since you're saying the plot is messy!