There are a couple of issues here that jump out. Firstly, I think a grandfather clause, if one would exist, would only apply to you if you had already applied for your GC, and the new law came into effect after your application date. Since you cannot apply for GC right away (can you?), at least, I presume, not till you are married any way, you may become subject to the new law when it comes into effect.
Secondly, you mention you met this girl in mcd - what is mcd? McDonald's? What are the chances that your relationship will last through to the point where your case gets adjudicated, which could be a few years from the date of application AFTER your marriage? With the divorce rate nowadays and the level of tolerance (or lack thereof) in the general populace in the US today, what are your chances that your case will still be valid if your relationship/marriage falls apart (god forbid)? Have you determined what your process for legalization is here? i.e. Fiancee Visa, then GC, then Citizenship etc.? What are the timelines and requirements for each step? What are the dependencies (i.s. staying together in a marriage, etc.). If things do not work out between you and your girlfriend, will you be forced to eat humble pie, againsst your will and try to stay in the relationship? considering that that would even be an option?
I would strongly recommend that you should seek a competent immigration attorney's assistance at this time to determine all your options and weigh your best course of action at this time. Your case has many facets to it and it is for these kind of cases that proper legal counsel should be sought rather than depending on advise from lay people (no offense to other users of this forum) on this forum. After all it is your life and future prospects that you are talking about here. Many immigration attorneys provide a first consultation free of charge.
Please keep in mind that I am not an attorney and it is just my opinion that I have expressed here. I wish you the best in your journey for ultimate permanent legal status in this great country.
Good luck!!