java projects

Hi! I’ve been working on a Telegram bot in Java, and I built a large architecture for the project, splitting it into microservices. I almost finished the code, but in the end, I realized that I made it for just one user. If the project is used by multiple people, they would all have access to the same data because I didn't account for multi-tenancy. Now I need to redo the project and make changes to a few microservices.

How can I avoid such major mistakes in design from the start? What can I do to anticipate these things during the development phase?

How do you approach designing your projects? What principles and approaches do you use to avoid similar mistakes in the future?

Thanks in advance for your advice!