Read the book Mathematical Thinking by Keith Devlin. Perhaps accompany this with the Coursera video lectures by the same.
The book contains a number of exercises that seem necessary to really understand the material.
Ideally I would find a study partner or group to do this book with.
Why Read This Book?
I was originally a math major, but life events caused me to delay my education until math was no longer a worthwhile choice. The ensuing decades have worn off whatever math veneer I had, and anything above linear algebra is fond memories.
This book promises to help bridge the gap between calculation math and conceptual math, which is what math starting at proofs really begins to be. And that is what I believe I need to understand the topics I’m most interested in.