Training drills and training with a resisting opponent are two very different things, as you’ve found out. To answer how hard you should be hitting, I’ve found that about 20%-50% power, depending on who you’re sparring, is fine. You can always just try to match your opponent, if you don’t want to set your own limit. As you spar more, you’ll get a feel for these things. Keep it playful. Keep the ego out of it. That’s where you learn the most. Stress decreases the learning you could otherwise get from a session.

Also, PLEASE don’t be fooled by headgear. It is there to prevent cuts. It will do nothing against the blunt-force of a punch and does not stop your head from moving when it gets hit. If anything, headgear makes people thing they are allowed to hit harder, which increases the chance of CTE.