Well, the answer to that is living in the moment. As a wise old Scotsman in my home group puts it. "If we go around with one foot in yesterday and the the other foot in tomorrow, sure enough you're going to pee all over today." (For added authenticity just insert a broad Scots accent).
It's unlikely that we will reach the stage where we dont let our thoughts drift to the past or future in unhelpful ways. The answer is not to stop it happening, but have a constant willingness to gently bring your thoughts back to the now. It's easy, when the feathers of fear are ruffled, to velcro ourselves to those fleeting thoughts because they seem important, or significant or US. But they are just thoughts. One AA speaker (Bob B I think) likened it to trains pulling into a station. Saying that "we can just watch some go by - we don't have to jump on every damned train that pulls in." I liked that one for some reason and have used it as a reminder a few times now.
BB