Here's what I think,
If your technically sound, it sounds like this is a mental problem, more than physical
Of course, keep sparring,

BUT I'm also with Lyle

Shadowbox, but next time you shadowbox IMAGINE your in there with a KILLER, imagine everybody's watching, someone's yelling at you, the pressure is on...

Thats exactly what musicians do when they practice alone, they imagine they are under pressure, under the spotlite, in front of thousands...
... so when the time comes to step up... you are ready MENTALLY.