My biggest eye opening moment with theory came some 15+ years ago when I was studying Jazz Improvisation @ a small L.A. music school
The teacher (an amazing musician named John Daversa) taught us how to dissect a song by knowing the key, knowing the chords and melody and understanding where the song would move in and out of key (so to speak) based on the chords and melody. This was also very helpful in teaching me about modes and when to play certain notes from certain scales, etc...
He used a few jazz standards as his song examples as they have a real basic structure, but they do take small twists and turns through the progression. Plus, they commonly have extended chords (7ths, 9ths, 13ths, etc) that help stretch your understanding beyond the basics.
From that I "got" theory beyond the basic I-IV-V progression.
Since theory is such a big elephant that you have to eat in small bites, start simple. For example, take C major diatonic scale and make a chord progression using basic 1-3-5 triads through the scale. Can you do this or is what I typed all greek to you?
