What textbook can I use to really understand microeconomics, macroeconomics and mathematical economics
Your question is quite broad, but here are a couple of (open) book sites you might get started with:
https://open.umn.edu/opentextbooks/SearchResults.aspx?subjectAreaId=4
http://www.freebookcentre.net/Business/Economics-Books.html

When you're familiar with the basics, you might ask the question again with a more specific topic you'd like to study.

Expertise on economics (or pretty much any academic subject) is gained through years of studying and thought. It's, at least to me, a fascinating way of thinking about the world.