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")
Focusing, which is an introspective technique, is explicitly not focused on solutions; it's focused on figuring out what the actual problem is (which generally is more about listening to the complaint than it is about thinking about the environment or how things could be solved). This then helps someone find a solution, but they're likely not doing that with Focusing.