I'm no pro, and I certainly can't advise you any better than Larinda re: what feeling you should chase to achieve the desired shape. But I've been focusing heavily on posture (and anatomy) myself for the last couple of years, so the topic really interests me and I can't help chiming in.
As an analytical type, what interested me most at first is how am I organizing my muscles and bones when I stand up properly. That's different from feelings--there is only one "correct" organization, but each individual will use different feelings to get there. Here are some things I've learned.
*******
The spine has four main sections in it, three of which matter for dance posture:
--Cervical (neck)
--Thoracic (ribs)
--Lumbar (between ribs and pelvis)
--Sacral (tail area. This is the one that doesn't really matter)
Each of these sections has a natural curve, the extent of which varies between individuals. The L and C spine are concave backwards, while the T spine is concave forward.
What distinguishes dance posture from everyday posture is that we are actively attempting to decrease the curvature of the C, T, and L spine. The "theoretical" ideal dance posture in the spine is one with almost no curvature. This makes you look maximally tall and long.
(note: this is purely theoretical! A purely straight spine is a REALLY dangerous thing--those natural curves are important for load-bearing and shock absorption. A number of professional ballet dancers actually suffer injuries related to overextended spines. But for amateurs like us, who are not sufficiently trained to the point where we are at risk of that, it's a useful rule-of-thumb to remember)
The feelings your teachers talk about are all oriented at straightening these three curves. For example, "chin down, head back" is one feeling you can use to extend the C spine. "Toned core, suck in the belly" is a feeling that you can use to extend the L spine. And "raised sternum" is a feeling that helps extend the T spine.
*******
One of the main problems I've had to overcome--which may also apply to you--is proper awareness of the T spine. When told to "raise my sternum", I often would do this incorrectly by bending backward slightly through the L spine. That's only natural; backwards is the way the L spine naturally bends! But this meant I was INCREASING the L curve, rather than extending the T curve. That gets very uncomfortable very fast, puts my weight in the wrong place, and makes me look like a misshapen hunchback. All kinds of bad stuff.
(I had to do a lot of stretching and weird body-weight exercises to gain good awareness of the muscles involved in extending the T spine, and my mobility there is still a work-in-progress. However, I was hindered by literal decades of doing sports where not only was thoracic mobility unimportant, but the exercises I was doing significantly decreased it. Hopefully, you are not in the same boat there, and will not have such a steep hill to climb)
By contrast, if you look at images of highly skilled dancers--especially followers--you will note that their spines have little perceptible curve, even in the lumbar and neck regions. Pay specially close attention to flatness of their upper backs (T spine); THAT is where the magical, backward-stretched illusion comes from.
*******
As for feelings, the one that worked for me is almost exactly what Larinda described. Listen to her! But also knowing what I was trying to accomplish mechanically helped me put that feeling into perspective, and use it more intelligently. Perhaps this will help you a little bit, too.