She's capable of studying any new topic much faster than I. Even topic that she has no experience in and I have been working on for several years. She's doing an MD in Medicine and a bsc in computer science simultaneously. She's also a Marathon runner and a quite successful artist.
Marathon runner - Result of LOTS of training; you shouldn't count this at all, except to say she's motivated to run.
Successful artist - Creative, skilled, and/or knows what people will buy. Are you a successful <insert job position here>? How did you get there? Surely you have some combination of skills that got you there.
Quickness of learning - Perhaps she knows the optimal learning style for herself, and you do not.
MD + BSc - Time management plus being able to retain the information and apply it. This could relate to the quickness of learning.
Now for the bad news: Maybe you're legitimately "not as smart" as your girlfriend, for whatever definition of "smart" you're using. Not everything is about pure IQ scores.
I agree with the other comments that people have different areas of skills, and everyone may not have the same ability level in a given area, but the idea is that your skills complement each other.
I am not sure she's more motivated than I. Perhaps she got better abilities to focus on tasks. She's truly an exceptional person with an exceptional range of abilities.