I have been playing blues guitar for a little while now and have a couple songs under my belt. However, the one song ive been dying to learn how to play, Sad Letter by Muddy Waters, still eludes me for this simple reason, i can't figure out the chord progression. I can hear it in the background but don't have a good enough ear to pick up what chords are being played.

If anyone here could help me out, I would be much appreciative.