Problems detecting slight pitchiness while singing

I am a female acoustic singer-songwriter in my late 50s who has been active in folk music (solo and duos/trios) for the past 10 years; I also sang folk for 3 years in my early 30s, rock (except for a few months after childbirth) until my early 40s, and took time off until restarting at 48. I was a choral soprano in my teens and 20s. I studied Stanley method from age 29-34 (but stopped both because I had a C-section and the method had begun to be discredited).

I always prided myself on a keen sense of pitch (and people often had me tune their guitars after they couldn't, even with electronic tuners--still do). After releasing my first CD in 2001, I was dissatisfied not with my intonation (which was fine) but with my vocal quality, which sounded tired and lacked an "edge" or clarity (not raspy or breathy, just "cloudy"). Sought out an SLS-method teacher who suspected a bowed vocal fold due to my hiatus from the stage, and sent me to a singer-specific ENT, who scoped me and confirmed the diagnosis. After months of exercises to strengthen chest voice, I was cured--my voice is much clearer, though I'll never sound as current or edgy as the modern crop of alt-folkies (but at least I don't sound operatic or "old").

Had a hearing scare in '04 when after a cold I woke up one morning and acoustic stringed instruments and pianos suddenly sounded weak, tinny and horridly off-key, though vocals sounded fine. Only way I got through my gigs was to amplify my guitar. Saw ENTs and a neurotologist, who tested my hearing (and pronounced it perfect for someone my age and superior for someone who'd played rock--he said I was just "fussy about pitch") and via MRI ruled out any kind of acoustic neuroma, patulous eustachian tube, or brain tumor. I had to beg him for a course of prednisone, which--given at the last possible moment along with Vitamin E, manganese and vinpocetine--cured me within less than a week. Noticed that for a few weeks I struggled slightly with pitch in the studio, but was able to tell as soon as it was happening and punched in to correct it successfully; even that soon cleared up. I have been religiously wearing my custom earplugs (-9,-15, and -25dB depending on the situation), though not onstage--because I know the sound of my voice via bone conduction is not what's going out to the audience, and I need a little reverb in the monitors to sing accurately.....provided I GET monitors, which in some clubs or in totally unplugged house concerts is not always the case

Then a couple of years ago, I noticed that sometimes when I sang onstage with anyone louder than me, I could immediately tell I was flatting. Solo live recordings sounded fine. Then in the past few months, I began to notice that I could not tell onstage that I was going flat until the notes actually came out of my mouth (by then it was too late) or even until playback, even when I used the workarounds of vowel pronunciation and placement and making sure my vibrato was kicking in naturally. And this past month, I've been in the studio and to my horror, sometimes I could "feel" myself flatting even on solos, not just harmonies; and occasionally not even hear it as I sang (even with my vocal volume up in the 'phones), but only on playback. In fact, the weird thing is that I can perceive pitchiness in my recorded vocals and other people's live and recorded voices--even the tiniest bit. I asked my engineer if I was having high-frequency hearing rolloff, but he said no, I was hearing just fine up to 13K and had only a dB or two less acuity than he did. (Bless him--he's been able to sense when I felt I needed a punch-in or judicious pitch correction). The frustrating thing is that my partner--a baritone a few years younger than me--has a severe high-frequency loss to the point that it shocked both me and my engineer, yet HE rarely goes off-key, never warms up or cools down, never does vocal exercises.................and he's a smoker!!!! (I never smoked). And of course he can't tell that I'm flatting......but I CAN, and I KNOW that critics probably can too but don't say anything.

I keep asking my teacher (I take lessons weekly) and he says he doesn't notice any intonation problems in my vocalizations or when he has me occasionally sing a new song I've written so he can advise me. Mostly, we just exercise and vocalize; should I seek out a vocal style coach in addition?

The problem never occurs in head voice, very rarely in middle, but mostly in chest, and most often on descending lines and notes at the end of them (even with plenty of breath to spare). Oddly, it never happens when I sing blues. I'm not a big fan of my soprano register because although it's sweet and clear, it doesn't sound particularly distinctive or have the character and identifiable timbre of my chest voice.

I am frustrated, embarrassed and mortified. I heard a live podcast I did a last month and was horrified how many clams (several cents flat) I was hitting. How come I can hear myself going flat on recordings, and other people's pitchiness both live and recorded, but I can't catch it in time or sometimes even tell WHILE I'm singing? Why is it happening in lower register? (Got scoped again last winter after a bout of allergic laryngitis--no bowed cord, just phlegm from postnasal drip).

Are there any exercises I can do to improve my intonation (as well as my perception of it as I sing)? Anything I can find online is geared to tone-deaf rank amateur newbies who can't even detect a semitone of difference between notes. Can this be due to obesity or dehydration? I hate to think I need to "hang it up" due to aging, especially since I still have fans (club owners and DJs among them). I also hate to think my pitchiness may be harming my singing partner's career too.