Performing well on the GMAT is indeed possible within your prospective time frame, but it will require a level of commitment and focus that, for whatever reasons, has been lacking in your previous preparation. Fortunately, you are probably knowlegeable about the GMAT, what to expect, which products to use, and so forth. In other words, you should be able to organize a solid study plan.
I certainly realize and appreciate that life can complicate one's GMAT preparation. Before getting started, then, it might be a good idea to schedule certain days and times that are solely devoted to the GMAT. Have that sit-down with your girlfriend and see whether you two can reach an understanding. Do your best to clearly explain your goals and what it will require to reach them. Likewise, listen to her and understand her test goals and input. And please do not become too stressed! Focusing on the whole thing (GMAT, relationship with said girlfriend at this time, school application, MBA, work, etc.) is just too much. Just take things one at a time and divide your overall preparation into digestible pieces.
Good luck!