As a general rule, if you need to prune, You will lose your flowers for next year.
However, you can lightly prune , leaving the buds, for next year still on the plant. This is because the new flowering buds are forming as the plant is in flower. You can see the new buds easily. this is best done when the flowers have finished. I hard prune mine every 4 years or so and miss a season of flowers.