What is wrong?

I've had two weeks of pretty much 90% baking failure (on a variety of breads) and I don't understand why.

Here's what I know isn't the problem:

*Insufficient gluten development (windowpane almost every time)

*Dough mixture (using same ingredients, not doing anything radically different from what I was a few weeks ago)

*Inactive starter (my starter is more active now than it has ever been).

*Oven (same oven as before - despite the problems I've had with it).

I've been focusing on shaping my bread and improving it. What I've found consistently is that with 60% or higher hydration doughs - regardless of how much tension and gluten development I have in the dough after first fermentation & primary shaping - after I leave the dough to proof a second time, the dough just flattens out into a puddle. Most of the recipes I see call for 60% or higher hydration and yet acheive perfect shape and crumb...HOW??? I've used stretch and fold, french fold, kneading in various combinations...also let ingredients autolyse after initial mixing.

Why is this happening? I don't think I'm overproofing (dough seems slightly firm to the touch)...I'm shaping better than I ever have before or at least that's how it feels initially BUT almost every time, after first proof and preshaping I find it hard or impossible to shape the final dough - it's just way too fluid regardless of hydration or gluten development. It may hold shape for 5 minutes before flattening out. This is becoming a serious annoyance because I can't figure out why this is happening. It's not like I'm doing anything obviously wrong. I'm trying to follow all the guidelines I've read (resting between preshape and final shape, maintaining tension, using the friction of the surface to develop tension etc.) but how am I ever going to improve if none of the followed advice (shaping batards, boules etc.) seems to work?

Additionally I'm getting little or no oven spring with the worst cases resulting in a solid glutinous mass.

I know I know, I should provide details, figures, photos - which I can't provide - there were quite a few different recipes/variations. Something more fundamental would seem to be going wrong.

Is there, for example, something in my sourdough starter that is essentially eating away at any gluten (or more importantly slackening the dough) I have developed?  I will say that the proofing times have been quite long of recent days (typically 6 to 8 hours at room temp for primary and sometimes almost as long for secondary).