Music theory isn't a simple thing. That's a field I'm always working on, so many things I don't understand. Here's a website that can help a lot.
The chordmaps you can find there are very interesting. Starting with simple chordmaps, until you find very complex ones. You can work on chord progressions, and suddenly you understand a lot more than before. How does a chordmap work? Just start your chord progression with Chord I. Then jump to any chord on the map and start your way back to chord I using the arrows. Try it out.