What should I do about being a 'push over'?

My girlfriend and I were speaking about a situation happening with her best friend and well the whole “what would you do if a guy was talking shit about me” conversation came up. Well she gave me honest answers and yeah I wasn’t too happy to hear them, but it’s okay, because all that really matters to me is that she trusts me enough to open up to me like that.

Well my issue is, in middle school, I was the push over as it may seem. Letting people do what they want, talk shit, all that. Me, I wouldn’t respond, I would just sit quietly and just ignore them. I got bullied, emotionally, verbally, physically. It just so happens that now and days I really could give two shits about what people think about me, or if they say something, I just ignore them and walk away. My girlfriend does not exactly like this and I would like to change this because it makes me seem like a ‘push over’. Any ideas?
Yeah I think it’s a waste of time fighting or arguing with someone for something they said, but if it gets to the point where they touch her in any way, that’s where I don’t tolerate it. But the problem is I am only like 5’3. Most guys are way taller than me, stronger than me perhaps, this is another conversation that came up, and the result is me working out. I know that height or strength has nothing to do with it as long as you know what you’re doing, but what do you suggest I do to be able to not be “beaten” by someone taller or stronger in case it does engage in a fight?