Advice on firing back whilst under pressure.

It kinda depends how old you are, but always avoid head damage as much as possible and spar way less. Going to technique, if you're a natural counter-striker watch guys like Lomachenko if only from a technical standpoint. The way he uses his jab and footwork sets up his shots and he's usually always first even though he's a counter striker and his footwork sets up his defense.

Guys like Lomachenko are suburb defensive fighters, awesome footwork etc, while still being very aggressive himself, especially with a jab. Your goal should be to master the jab and footwork, most don't and from what I've studied in strikers, those are probably the two biggest tools in striking. 

If you want to throw caution to the wind and you think you have a good enough chin, it might help to study guys like Ray Sefo or Mark Hunt, who tend to take a shot or two to close the distance and land their bigger shots. If you don't have the power in your shots yet, wait until you grow into your body and decide your style at that point later on. Maybe a hybrid style.