When attacked the choice is not yours.
I once attended a self-defense class at the library led by a police officer. If you are looking for a class to take check with the local police department or the community center and see when they offer them. If they don't, ask for them to do so. But during the one I attended the officer spoke about a time he was asked, “How can you choose to take a life?” It was in reference to his job and carrying a gun. His answer was simple, “I don’t make the choice. The other person does.”
When you are walking down the street you do not decide, “I’m going to attack this person for the fun of it.” However, someone else does make that decision when they attack you. At that point they have also made the decision to deal with the consequences, whatever they are. You are free to defend yourself. You don't want to become the attacker, but whatever it takes for you to be able to get to safety is what you need to do. You are responding to a choice someone else has made.
People seem to either understand this concept or struggle with it. I am one who struggled with it. When I first began learning self-defense moves and sparring in my taekwondo class I was scared to death I was going to hurt someone. I was fearful of being hurt as well. I have learned while practicing when done correctly the moves we do we will not hurt one another, and the gear we wear while sparring protects us. But, when done correctly in a self-defense situation I can protect myself and create a way for me to leave the situation and stay safe. This is why self-defense classes are so great. They give you a safe place to learn techniques which will allow you to respond to someone else's bad choice.
Always remember, the person who is making the choice is the attacker. It is more than okay for you to defend yourself. Yes, they think they have the upper hand, and they think you will be the one to receive the consequences of their choice. Make it so that isn’t the case.