A reformed free-spirit spender's perspective:
When someone isn't on board with frugality, the problem is simple: they don't have a clear vision about WHY they're saving money.
I used to be just like your girlfriend. I met my tightwad husband, we got married, and he was coming to me saying, "We need to save money!" I was like, "Cool! For what?" His answers never did anything for me. "Retirement" to me, at that time, sounded like the absence of work. That image is only inspiring to me when I'm in need of a nap.
My husband then made a common tightwad mistake: he thought explaining HIS vision to me - over and over and over, in many different terms - would eventually inspire me. It didn't/doesn't. We are way too different for me to be inspired by his goals.
My attitude changed when we both put some serious effort into establishing a vision for my future. Try asking her this question and see where the conversation takes you:
"What would you do with your time if you could do anything at all? If money was no object, and you didn't have to spend 65 hours a week either getting ready to chase money, chasing money, or recovering from having just chased money, what sorts of things would you do with your day? Create that perfect day in your mind." Then move on to "perfect life." (Whatever you do, don't judge her comments out loud!)
Get her talking and keep her talking about HER wants and needs for the next month or so. I know it sounds dangerous to ask a spender to discuss their wants but she needs to start thinking about the underlying WHY of saving. She may not know the answer to the question. It took me weeks to figure it out. I'd spent years just floating through life with no goals. But once I did, once I got a clear image of how I'd design my life if I didn't have to give away my time in order to earn a living, my attitude changed. Right then. When my husband turned to me and said, "We can make all that happen if we do this: ..." I got on board. And every time we pass a milestone it gets easier. I still want to spend money on random junk sometimes, but now there's something I want more. And you can't do what some people here are suggesting and somehow magically pass YOUR wants, needs and dreams on to her. She needs to grow her own vision, and become aware through you that its within her reach. THAT'S your role.
One last thing: don't get caught up in trying to make your dreams match. People mature and change with time, so neither of your dreams are carved in stone. The important thing is that each of you have what you personally need in order to want to lay money aside. Once we have half a million dollars set aside we can sit around and argue over whether to move to a goat-farming commune.