Sorry you haven't had any replies, I don't think from what you've said you actually need to change your behaviour that much, no-one aside from you has actually said you come across child-like, it's all about your own confidence and how you feel internally? As in, you feel your DH is embarrassed by your small talk, but he hasn't said so (surely all small talk is a bit cringey anyway, once it moves past that stilted part it stops being small talk and is actually just conversation)? You feel people are looking at you when you talk in meetings but you haven't had any feedback from your manager or colleagues that you need to improve?
Have you had any therapy/counselling about the childhood issues and confidence/imposter syndrome etc? Sorry if you've already done all that but you don't mention it, if you find the right person it can be really helpful.
It's not exactly the same but I've suffered with really quite bad shyness a lot of my life, and was actually getting both personal and professional negative feedback about it so I had to embark on a bit of an improvement project. I didn't really do anything ground-breaking, just practiced, a lot - so putting myself into scary situations where I would have to make small talk with other shy people (my nightmare even now lol), have to speak publicly, have to assert myself in conflict-y type situations, that kind of thing. I took inspiration from people around me who do that kind of thing really well and tried to copy what they do as best I could (I know people say you just have to be yourself but myself is a frightened mouse who'd happily sit in a corner and not say a word in a social situation and that wasn't working well for me 
), so I kind of did the classic fake it til you make it thing. I also did an interesting course on how to be a good listener, I'd always thought because I was so quiet and let others do the talking I was a good listener but actually there's more to it that that, learning how to get other people to talk well and 'feel' listened to is a super useful skill and once you know how becomes kind of second nature and is really handy because most people prefer to talk about themselves than to listen to you talking anyway, and so saves you from having to say too much! That course was way more helpful to me that the classic 'assertiveness', 'public speaking' ones that work also put me on too although those were also OK in that they taught me there's no real mystery about how people get good at this stuff, it's just technique (fairly obvious technique too) and practice...
In my profession we regularly have to do 360 feedback and I now no longer get the constant 'too quiet', 'doesn't speak up', 'is shy' kind of comments so professionally at least it's worked, and in personal life I've pretty much stopped caring what people think of me, I can roll out reasonable chat in any situation that really needs it (the good old stop-gaps of the weather, the state of the roads/traffic and what's on the telly have served me very well for many a haircut/visit to elderly relative over the years, boring maybe but saves lapses into silence) and frankly unless you are saying something offensive or unkind why does it matter what your small talk is? If your DH is embarrassed by it you have a DH problem not a you problem (to roll out a MN classic)?