First, is your girlfriend a US Citizen? If she is, I would advise you to marry and file the I130/485 asap. Of course, you will have to prove that your marriage was entered into in good faith. If you were legally admitted into the US and overstayed, as long as you're married to a US Citizen (in good faith), that can be pardoned. Of course, since you are currently in violation of the law, I suggest you do this quickly.
Further, if you leave, you are not ensured you will get a B1 visa. You may no longer be eligible for the VWP, so you'd be taking that risk. In the case that you were denied re-entry, you would have to remain outside the United States and consular process after you married.
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There is no specific prohibition that you cannot file a spousal petition as an overstay on your VWP; however, if you decide to proceed this way, you would get married and file right away. Essentially the decision whether your I-130 will be granted will be at the discretion of USCIS officer and the strength of your application. Consult an experienced immigration attorney to help you with this.
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I agree with my colleagues. You should also be prepared to prove that you did not have a preconceived intent to marry at the time of your entry. I strongly suggest you to consult with an immigration attorney and have your individual case and documents evaluated. Because, some jurisdictions give really hard time to VW entrants who file for Adjustment of Status and if you receive a denial and put into proceedings you cannot ask for Immigration Court review since you waived all your rights at the time of the entry.
I agree with my colleagues.
J Charles Ferrari Eng & Nishimura 213.622.2255 The statement above is general in nature and does not constitute legal advice, as not all the facts are known. You should retain an attorney to review all the facts specific to your case in order to receive advise specific to your case. The statement above does not create an attorney/client relationship. Answers on Avvo can only be general ones, as specific answers would require knowledge of all the facts. As such, they may or may not apply to the question.
No need to worry. Assuming that you marry a US citizen, all will be fine. Two month work permit processing and three month green card processing! And there is no penalty for overstaying a visa waiver. Do not leave the country. If you did this, it would take about a year to come back after your marriage. It is an easy process as long as you stay put and find a good immigration attorney : ). By the way, this is the only type of immigration case that I handle. This is my specialty!!!