Learning to strike somebody and learning how to take a strike are essential.
Being hit can really mess you up.
Clever self defence tactics and techniques may fall to pieces when you are actually taking hits.
It is imperative that you know how to relax and roll with the punch.

Developing your own striking ability is critical. Without it, you cannot hope to defend yourself.
You need to make each blow count. 
Range, timing, distance, accuracy and penetration must be practiced relentlessly.
A bag or focus mitt is not the same as a person. You need to strike real people.
Do not pull your punches. Let them land. Feel whether or not you are receiving adverse feedback.
Learn to control your power, your commitment and your intent.

Punching thin air may train the body mechanics behind a strike but tells you nothing about your ability.

Fear needs to be addressed as well as competence under pressure.

You do not have to endure severe combat training in order to see how you cope under duress.
It can be done safely and playfully. If this sounds improbable, look at sport.

What matters is the core requirements:

1. Keeping a cool head
   - no anger
   - no upset
   - no panic
    
2. Decisiveness
   - action
   - no dithering
   - no getting ready/anticipating
   - no worrying
    
3. Effectiveness
   - does it work?
   - are you able to follow it up?
   - can you improvise, adapt and change?

You need to know that the material works and have the confidence to use it when the situation is unpredictable and confusing.