As a romantic suspense author, I spend a lot of time writing about ladies in peril. And by that token, I have to get them back out of it. So I'm going to use one day a week to talk about self defense and protecting yourself in various ways.
You never want to need to defend yourself.
If you're ever at the point where you actually need to fight an attacker, you are working at a severe disadvantage. Your opponent has a plan - you don't. Your opponent has the advantage of surprise - you're working several steps behind them. Your opponent has probably struck first and may have weapons, and even if for some reason you were carrying a weapon, it's unlikely you'll have the time to get it out.
There are a lot of self-defense classes out there that offer training in mostly martial arts. While I'm not saying these are a bad idea, they're a lot more useful in terms of cardiovascular fitness than they will be to you should you ever be attacked.
A class situation, operating under rules and limits, is completely different than an attack that takes you by surprise. And your own capabilities are different during this kind of fight - your adrenaline will kick up past the point where it's useful, you'll probably develop hyperfocus, and you won't be able to think with the same logic that you normally use. The chemical cocktail that affects you during a fight will make each person act slightly differently under its influence. Some common side affects include loss of peripheral vision, degradation of fine motor skills, and an obsession with irrelevant thoughts.
The only way to reliably overcome these disadvantages is through training (and even then, there's no magic bullet, so to speak). Presumably you aren't getting training for special ops, or you wouldn't be reading this blog right now, you'd be out there throwing boats around and ripping trees up from the ground. That means that your best bet is to avoid situations where you have to defend yourself.
If a situation looks strange to you, leave. Don't worry about hurting people's feelings or acting odd - have you ever heard stories about soldiers who come home and hit the deck when loud noises go off, like books dropping off a desk? They've learned the hard way that it's better to be safe than sorry. Follow that example, not often nonsensical social rules. Trust your instincts.