After the Mexican-American War, Mexico was obviously extremely angry at the U.S. for taking about a third of their land. Now, however, the two countries are awfully friendly towards one another. So when did Mexico "forgive" the U.S?
To put this into perspective, let us go back to 1848. Mexico City has been occupied by the U.S. and it has taken a large amount of territory. At this time, there were those who supported Mexican Republicanism, later to be led by President Benito Juarez. There were others, such as members of the military, the wealthy, and the Church, who wanted to reinvigorate the Mexican Empire that existed prior to 1825.
In 1862, Emperor Napoleon III of France invaded Mexico and instilled the Austrian Maximillian as Emperor of Mexico. Abraham Lincoln was sympathetic to the Mexican Republic under Juarez, but was unable to help due to the Civil War. Juarez eventually succeeded and reinstated the Republic.
Fast-forward to 1910 at the start of the Mexican Revolution. Two of the most important figures in that event were Victoriano Huerta and Venustiano Carranza. The United States was known for its opposition to Huerta's regime due toGerman attempts to illegally arm Huerta's forces. This was one of two times that Americans would violate Mexican sovereignty during the Revolution.
Another notable figure was Pancho Villa, who opposed both Huerta and Carranza. After he raided Columbus, New Mexico in 1916, American forces under Gen. John Pershing sought him out in the mountains of Northern Mexico. Once again, this breach of borders angered the Mexican government. This was further exacerbated by the Zimmerman Telegram in 1917 in which Germany promised to help Mexico get back Texas, New Mexico, and Arizona if it agreed to attack the mainland United States during WWI.
As for when Mexico "forgave" the United States, I am not aware of any specific event. Over time, Mexico and the U.S. decided to embrace each other as economic and political neighbors. A lot of credit can be attributed to Bill Clinton and his work with Mexico on NAFTA, but in the case of relations, it is fair to say that economics has bridged any major political differences the countries may have had in the past.
As /u/anthonyvardiz says, a lot of it could probably be attributed to NAFTA, but I think its worth noting that relations aren't perfect. There was a lot of uproar following the whole ATF gun-walking scandal and the Drug War/Border Security/Cartels/Firearms mess continues to strain US-Mexico relations.