I had a very similar experience. Somewhere in my life, I started to swallow my voice, especially my singing voice. This came off as my tone being nasal while we sang. I've had off and on luck with correcting it. Sometimes I sing openly and beautifully, and think I've got the problem licked, only to notice later that I'm swallowing it again. I'm horribly fickle on exercises my therapist has suggested to help with it, so I end up feeling self-conscious about it, which, of course, only makes it worse.

So... I can't offer much help, but I can offer much empathy and sympathy, for what it's worth.