I've done a bunch of (what seems like) introspecting a bunch of (what seems like) ruminating. To me, there is a distinct difference between the two, but ironically I think telling the difference requires you to have some skill at introspection.
I could point to concrete differences, such as:
- Rumination is repetitive.
- Rumination is more like a rehearsal. It feels more like I'm saying words in my head.
- Introspection is more like listening than saying things.
- I (usually) feel like I ruminate by accident, and I introspect on purpose (although I can imagine a high-level mindfulness practitioner introspecting by reflex)
But ultimately it's more like "Rumination has a different flavor than introspection." Sometimes introspection is goal directed, sometimes it's not. Sometimes introspection is repetitive. Sometimes it's not. The rules I can come up with for "what is introspection?" don't feel ironclad.
(Except perhaps that it feels more like listening than talking/acting?)
But nonetheless, when I'm ruminating, my whole body usually feels like it has a particular stance – more stressed, more concerned with action (sometimes rehearsing actions I plan to take later, sometimes just worrying about the state of the world as it is now). Whereas introspection feels more tranquil, like I'm sitting by a lake and observing it.
And maybe (if I'm having a bad day) the lake is full of garbage and the sky is stormy. But my stance towards the lake isn't trying to fight anything, it's just sitting and noting "ah, I see the lake is full of garbage today." Whereas if I were ruminating I'd be thinking "gah the lake is full of garbage the lake is full of garbage I want it not to be full of garbage the lake is full of garbage."
(And yes, in that example the rumination thoughts are repetitive, but I think the key ingredient is beginning with "gah" instead of "ah")