Most Helpful Guy
Take up a martial art. An external and an internal one, and see which one suits you better. I have barely any muscle mass at all, but I'll bet I can hit, twice, if not three times as hard as people who are significantly bigger than me. Not that that will necessarily help much in a fight, especially since usually, if people start on you, it's because they're confident they can win, ie much bigger than you, or a whole group against one or two.
So it's not the actual fighting I espouse. Any good martial art teaches you how not to fight. It's about having confidence in yourself, seeing the situation for what it is, to have the confidence to walk away. Because in reality, those situations are an interaction between two egos. It's because either party feels threatened, capable of being trampled over by another, like they have something to prove, totally unconscious, that they are willing to fight. People who are content in themselves don't fight, unless it's an absolute last resort. So you need to have that awareness to see your own insecurities, and not hide from them. That's why I like Kung Fu, with their underpinning philosophies.
Anyway, you should take basic steps for both parties safety. Have a chat with your girlfriend. Don't go into certain areas at certain times. Because, as I say, when it comes down to it, being a good fighter, or being strong, still won't help that much against a weapon or 4 to 1.