How did you start the conversation with your girlfriend? I am having trouble getting my wife on board. She has never had to think about money before and it's completely overwhelming to her. I have read through the 50 things for the SO, and am trying to implement some of them. Also my insistent talking about FI and excitement over it doesn't exactly help.
Well, part of your problem with talking to her is she hasn't started working yet. I know if someone had talked to me about all of this in college, I would've rolled my eyes and told them how I was going to be a super rich baller living in a downtown loft. Once I actually started working, I found out that the reality of working 40+ hours EVERY SINGLE WEEK for 40 YEARS is not how I want to live. It's one of those things that you know is going to be tough, everyone tells you is going to be tough, but the gravity of it doesn't really hit you until you're doing it. I had summer jobs and internships, but they were always temporary. About my 5th month at work I remember thinking "So I'm supposed to do THIS for 40 years, holy shit this sucks."
My girlfriend was already somewhat frugal when I met her, but she spent a ton on her family and friends. Fancy meals, buying them presents, more fancy meals. Mostly it was food she was buying. I ran the numbers on her loans (which she hated), and told her if she cut back on some of the spending she could pay them off in a year or two. She was skeptical, but then surprised when I showed her the numbers. A year later, she'd paid off her $20k in student loans. That was 2-3 years ago. That was where she realized saving a lot of money was actually possible (unlike what the media leads you to believe).
In the last year, she's been a little bummed about our 1 bedroom apartment, and the fact that we don't go out to eat as much as she used to. One thing that helped, was we took a trip to another state and had a really fun long weekend, and probably spent more than we should've. At the end of it, I reminded her this sort of travel is a big part of wanting FIRE. It's not that I don't want to spend any money and live like a miser, it's that I want to spend money on the most fun things that will actually make us happy. Rather than spending $300/month on restaurants, maybe we spend $100/month, and take a $2400 trip every year. We still get the restaurant experience, but don't make it a routine thing that ends up being the "new normal" that we don't appreciate.
In the most recent conversation she was asking about our budget for early retirement. It turns out she thought I was only planning on saving enough for us to live in a tiny apartment, never go to restaurants, and never take trips. She thought I was saving for necessities only and planned to quit after that. When I told her the numbers, and that it'd be based on what we spend now and nearer retirement, she had a bigtime relief. Having an extra $1000 or so a year for restaurants, and enough for a nice trip or two helped put her at ease. She also asked if she could just work part time to pay for extra things, which obviously is doable.
A big part of it for you will be finding what motivates your wife. Maybe she loves her work, but also loves having time to travel. FIRE lets her have leverage in making her own hours. Maybe she is kind of a control freak and doesn't like having decisions made for her, FIRE gives you a lot of freedom. Maybe she wants to stay home with kids someday, and FIRE will let you/her do that.
Leading by example is also good. Save up a lot of money, and show her the numbers. You can also try with showing her the "here's how much this generates per year" using the 4% rule. Save some of your "fun budget" and still have a fun time. Do fun activities with her that aren't super expensive. Take her on a surprise trip sometime in the next year or two using CC rewards points. Show her that a life of moderate frugality can be really great. New cars and cell phones don't make people happy for more than a few days (or maybe weeks if they're lucky). Memories last a lifetime.