I know exactly what you mean. There was a time when I didn't pass at all, and I was painfully aware of it. Then I started passing but I didn't really believe it. Then I had the full ffs works and people wouldn't believe me when I told them I was trans. I would do things like hang out with people I knew from before that knew about my transition but I didn't tell them who I was and they had no idea I was actually someone they knew fairly well. Experiences like these made me very confidant in my passing abilities. Then life moves on and any real feedback diminished to almost nothing. No-one would stare, point or whisper. There are double-takes and no-one ever say anything. But there's no positive feedback telling me I'm passing either, so I've developed this intermittent paranoia that people are just keeping their mouths shut to be nice. It's really hard to shake this feeling that I'm not really passing. I knew I wasn't passing back when I for sure didn't, but it's been so long so I can't really remember what it was like.
About 2 years ago I took a work related 8 week course and was in a class of about 20 other students ranging in age from 20 something up to 50 something, and I couldn't for the life of me figure out if they knew or not. There was absolutely no sign that anyone knew, but no sign that they didn't know either. Every day I would sit in class and wonder if they knew or not.
The only way I can get over it is when something happens that make me 100% sure I am passing, and usually it takes a situation in which there is a good chance I will get clocked, like for instance being examined by a doctor or nurse. This happened two years ago when I fell over my bike and got the end of the handlebars right in my belly. I was sure I had internal organ damage and crawled into the ER examination room in jeans, t-shirt, ponytail and no makeup. The doctor examining me wondered if I happened to be pregnant because that could be serious. After that I felt pretty confidant for a while, but then again nothing happens and I slip back into the irrational pattern of thinking everyone I meet knows. The rational part of my brain is telling me I am passing, but I still doubt it for some reason.
Like Apples say, people have a hard time being honest, but in my opinion people that knew you well before your transition are just as bad at telling if you pass or not as you are yourself. One time I was hanging out with my brother and an old friend of our. My friend had never really understood my transition but eventually came to accept it. He still sort of saw me as the same as before, but then his ex that I knew pretty well came by to visit and she could not figure out who I was. First she though I was my brothers gf, then a few minutes later she was asking my brother about me and how I was doing being completely clueless I was sitting there right in front of her. My friend was rolling around on the floor laughing and was completely blown away that his ex could not figure me out, because when he looked at me he still sort of saw the same person as before. I think strangers are the only real measure of how passable we are or not.
I don't go out to clubs and bars so much anymore, but every now and then I need to to get some confirmation. Teenage girls scare the crap out of me, but then going into a club-restroom jam packed with them and having non of them react in any sort of way to my presence works wonders for my confidence. It's scary as hell at first, but it works. Sometimes you have to push the envelope to really know you are passing for real.
Anyways, if nothing indicates you aren't passing, you probably are.