I think it's hard to replicate the adrenalin rush before a race, but you can practice to control your breathing by swimming very fast laps. When swimming as fast as possible, the biggest problem is often to keep your breathing under control and to resist the urge to just stick your head out of the water.

I'm currently training for fast 50m laps (freestyle and breaststroke) and despite being a very confident swimmer I get a little panicky at the end of the lap if I do it as fast as possible.