Hi Donny,
First off, as I'm sure you know, sparring and fighting are two different animals. The former is to prepare and hone various attributes with no thought to winning or losing; while the latter is about WINNING and actually doing your best to knockout your opponent whenever you can. With sparring, it's okay to "hold back" relatively speaking because the objective is to build/refine your game, not KO your sparring partners or vice-versa. Now, when it comes to the actual fight, it's time to unleash. When I say "unleash," I'm not talking about going out and fighting like an out-of-control maniac, throwing caution to the wind. I'm talking about FOCUSED AGGRESSION. Piggybacking off of the good advice Trainer Monkey gave you,
This is probably not the awnser you want,but its a bit of both
You set up your bombs by manuevering the guy to where you want him,and then you drop the cannon
be "measured" and apply your game plan(s) skillfully AND aggressively (the two can/do go together synergistically) then go "ferocious" when the opportunities present themselves to do so. And when they do, these are the times, if tactically appropriate, to unleash your "aggressive volume punching" to finish off an opponent. Whenever doing so, you should be completely uninhibited about it. You shouldn't have any hesitation or deliberation to unreservedly knockout an opponent when you can. Remember, if the roles were reversed, what do you think your opponent would be trying to do to you?

If you have a problem "letting go," I suggest seeking the counsel of a sports psychologist (or, if you are self-disciplined enough, buy a good book) who can teach you neuro-associative and visualization skills to alter your mindset so that you can "let go" and do so skillfully. As you've found out first hand, "holding back" in this sport can get you unnecessarily hurt more...

Good luck ("luck" meaning when preparation meets opportunity).

Take Care,
Lito