No, I'm not referring to some B movie sci fi starring John Agar. I mean the fight between what we KNOW and what we SEE...it can be an epic struggle while painting. There's a tendency to paint outlines that fade into shadow because we know the edge is there - even if we don't see it. Yes, you're right...if you look at that edge you will see it. But that's because your looking at it...Confused yet?
Wherever the eye "slithers" to, that area snaps into full focus. Whatever the lighting conditions, your eye adjusts. The brain finds these edges and says "Aha! Hiding is futile! Prepare to be rendered!
What I'm trying to say is "don't find your lost edges". It's natural to look at whatever you're trying to paint...but remember it comes into focus when you do. If it's not the focal point of your picture then you have to see it as it appears when you DO look at your focal point. Sometimes that means part of it melts into shadow. Many people are reluctant to paint one object into another or its background. They miss how multiple objects (or parts of objects and their surroundings) can become a single composite shape. Their brain overlord tells them "you know these are discrete things. You must define them". It controls their seeing so it's not their own.
We must rebel and overthrow this tyrannical reign...
...boy, I need to rent some different movies! Let's look at some examples instead. Even those these teapots are photos, we can "find" the edges in shadow on the color example. The B&W image makes the lost edge more obvious.
When you're walking around, look for these lost edges. This may mean you have to turn off the general illumination found in most homes and use the ambient light from your windows, 19th century style (and we wonder why that era produced so many good painters).