Lynn's suggestion is great. You can add on to that.
But basically the concept is there. The reason why you are so much less productive than before is because you have yet worked out a mutually agreeable and understandable model of living together.
So you are working from home? You may need to let your gf understand that you do love her company, but you need to make ends meet and by that, it means you need to have a block of uninterrupted time (as what Lynn suggested) so that you can focus.
Let her know that by giving you that time undisturbed, you'll be able to finish it earlier because you're focusing, and you'll have more quality time to spend with her!
But if you are naturally distracted with her walking around, maybe you can work in a room instead of in the hall?
Good luck Steven! This is just a transition period from staying alone to having one more person under one roof. Use love to communicate and I'm sure you guys will work things out!
