this is my gut reaction: consider acting lessons, or joining a drama/theatre group of some sort. seriously - i'm convinced i can do all the stuff you describe cos i was an actor. it never becomes "natural" as such, but it does become second nature, and only takes a little effort, far less than you're making now. and it works - even people who think they know me well find it difficult to accept that i'm a spaz.
to me, all social interaction is acting anyway, except for that with people i'm close to (few and far between, probably only a handful).
it is exhausting, but you're loads younger than me, so you should have more energy. and it gets a hell of a lot easier with practise.
hope that's helpful. if i think of anything else, i'll let you know.