LEGAL HELP! CAN SOMEONE TAKE BACK PROPERTY THAT WAS GIVIN AS A GIFT?

about a year ago my ex gave me some land and she wrote on a letter that she gave it to me and a percentage of the land if it was ever sold. we got the letter notarized and since then i put tons of money into the property. now she wants to say the letter doesnt mean anything and its not my property. whats goin on?