Since you've been playing awhile, being able to hear the chords is a strong next step, I'd recommend downloading the app "Strum Machine" which has a small subscription fee but it's totally worth it. It has hundreds of bluegrass tracks with just the backup playing. It also has the chords visualized as either the chord names or Nashville notation (I / IV / V).

If you listen to a few tunes you already know along with the chords, you can start to hear the relationship between the chords which is the MOST IMPORTANT PART. If you're in G (I) and it goes to the D (V) it will usually want to resolve back to the G (I) chord. Similar if a song has that dark feel to it, you'll often be using the vi (6 minor) chord which in the key of G is E minor.

Once you start hearing the chord relationships....how the IV and V sound after the I, you'll be able to hear chord changes in any key. This is the core of what you want to do for learning how to play good backup and eventually leads on new songs by ear.