The openings are fairly practiced. I find there is a point where the understanding rapidly runs out and the little bit of claculation and intuition kicks in. I find it a bit like golf ... bear with me! The more you play golf you get better get up until you think your doing well then the inexplicable feeling of what am I doing sets in and the game goes downhill. After a while you recognise your problems, humility sets in and you gain another level of understanding.
Also, some time you play matchplay, other times stroke play ... they're quite different games. In the same way blitz, standard and competition all have different demands.
If only I had the surity and knowledge of those great masters that you pour over their games, understand why they did what they did and then can't reproduce it during a game!
Such is life.