Learning Polish help!?

Hi, I am interested in learning a new language. I am currently fluent in both Spanish and English, but I would like to learn a Slavic language. I think Polish would be perfect. I currently have a Polish girlfriend so she would be able to help me out. Also, I seem to be picking it up quickly, and some stuff I can understand even though I haven't actually tried to learn it. Many times, people who are fluent in Polish say that I sound almost fluent. I was wondering how hard would it be?