It bothers you a great deal about these things to do with etiquette. It might be a good idea for you yourself to think if you would like to take some etiquette classes, elocution, or whatever, because it's no big deal, and I'm sure such training exists, and even be fun.
You would feel positive and empowered. I don't like to see you being so unsure of yourself. Believe that this class stuff is all superficial nonsense in this age we live in, and you can base your style on any code you want and learn the archaic rules.
But, I don't think it will make you happy to be superficial and bothered about status.
Personally, I don't aspire to be posh, but if you do - feel encouraged that you, me, anyone can learn all this nonsense that bothers us, about which knife to use, etc. Lots of posters on Mumsnet have been hard up, then have become rich. Don't think you are always going to be the one who doesn't know the social rules of the group's you want to feel part off. Just learn it.
I don't like to see you feeling as if there is nothing you can do about the situation, but wait to see if you'll be accepted - or to feel that you are not in control.
Try to take ownership of social polishing, but do it for yourself because you would like to talk like upper middle class people. Accept it is your choice because you like it, not because you ought to make yourself different to what you are.