I came to the U.S. a couple of months ago on a fiancé visa (K-1), but as soon as I met my U.S. fiance's family I realized it wasn't going to work out. (His mother lives two blocks away, and tells him what to do all the time.) But an old boyfriend of mine happens to live in the U.S., and we've been in touch, and our romance has restarted very quickly. If that works out and we get married, can I get a U.S. green card? (He was born in the U.S.A.)
In theory, yes, you can get a U.S. green card through marriage to your old-and-now-new boyfriend, but you will have to be careful to do everything right procedurally.
First off, do not overstay your 90-day visa thinking that a new marriage will take care of everything. Someone who enters the U.S. on a K-1 fiance visa and then doesn't follow through with the wedding to the original petitioner has no right to request a change to another visa status nor an adjustment of status to U.S. residence. That doesn't mean that those statuses are out of your reach forever – just that you will have to leave the U.S. before asking for them. (See further explanation in Nolo's article, "Can You Renew or Extend Your K-1 Fiance Visa Status?")
You should leave the U.S. within the allotted 90 days. If you stay longer, your immigration record will show "unlawful presence," which can lead to penalties, in the form of bars upon returning to the United States. It will also make the immigration officials less likely to trust you when trust is needed.
After you have left the U.S., you can return either on another fiancé visa filed by your current boyfriend, or a marriage-based immigrant visa if the two of you marry before he files the visa petition on your behalf. For more information, see the articles in the "Marriage-Based Visas and Green Cards" section of Nolo's website, or consult with an experienced immigration attorney.