Aitana

Hi. I am spanish, and I have been dating for 2 years an american citizen. We have been travelling to visit each other for 1 year and a half, and then I came to the US for 3 months (visa waiver program). I came back to Spain, I stayed 1 month and came back here. They asked me a lot of questions but they finally let me in. I stayed 3 months more and I went back to Spain again. This time when I came back to the US they didn´t ask me one thing. I need to get a job. What is the smartest thing to do? Get married? Should we do this while I am here(till december)? Will I be able to stay here then, and apply for a work visa?
THANKS A LOT FOR YOUR HELP!