Would you have been interested in Islam before you met your girlfriend?
Was it an "aha" moment or a "I want" moment? (I know you can say both, but I believe there is a difference)
That, and many times in our religions and lifestyle in general, we don't practice all the way or maybe don't have the discipline to practice. BUT if the religion is there in your heart, you may struggle, you may fall away, but you can't just "not believe." Make sure Islam is a Fact for you not a belief.
How to become a Muslim
How to Convert to Islam and Become a Muslim
I know you're saying you're doing this for you. I don't see anything wrong with your girlfriend being an influence of that. Does not devalue that it's from within. That said, I wouldn't do it right away.
If you want to hear my experience (if you haven't already read somewhere on this site), when I met my roman catholic friend, she had a deep impact on me with her belief. I heard about god and studied what people say about god, so I went to the priest and asked him questions. "He told me to wait." I didn't. Went through confirmation and practice for four years. I love the discipline. I love the practice. I love the community, friends, and huge resources the church provides and supports you with. I love how they help people with charity (and how they helped me with food, clothes, furniture, and rent).
But at the end, that's not the reasons why I should believe in christianity. Not for friends (though they are influences), not for what I like to practice (and it was wholehearted), and not for necessity (food, furniture, etc). But for Me.
I hope you don't leave later on. I know there's a lot of hard things about Islam but that's just like many abrahamic religions. So, if you're going through with it make sure, in my opinion, it's something you've wanted and something you would continue to do with or without your girlfriend's support.