I would recommend this book from Keith Wyatt who is a great guitar teacher.

https://www.amazon.com/Harmony-Theory-Essential-Musicians-In...

I went through a lot of resources and this is the one that worked for me.

My advice would be to pick one good source, stick to it, and spend time to learn the material. It's easy but one has to devote some time and work. Use a pen and paper, do the exercices, be focused.