Design a custom ERP database with huge data that doesn't slow down

I have been working on my application for the last 15 years, but I have limited knowledge about database normalization. As a result, the application has become slower and less efficient. I would like to redesign the database to work with an API and also consider implementing double entry accounting. I am looking for someone who is able to work with me on this project, as I have a lot of ideas and concepts to discuss.