Once your 90 day stay is over you will be removable. While understandably the separation from your loved one is difficult, your timely departure is a prudent thing to do. If you do go back to your country and enter as J au pair, you could stay in the US to be near your boyfriend.
You shouldn't overstay the Visa Waiver Program or else you are immediately deportable without having the right to see an immigration judge. You should return to the EU before the end of the 90 days, or else you will lose the ability to participate in the program. From your home country, you can apply for a visa to come back to the US. If you get a tourist visa, you'd be eligible for 6 month stays with possible extensions, but since you intend to marry your boyfriend, you would not be able adjust your status in the US to get a green card later on. You would have to leave again for the interview at the US consulate in your home country. You should discuss this with an attorney to understand the details.