By mistake I ate a Schnitzel sandwich today and only realized after I had eaten it that it’s the nine days. I feel terrible and want to know how I could do Teshuva.
You should remove your mind from this completely and serve Hashem in a happy manner. It should be noted that according to the strict Halacha one may eat meat in the nine days. The custom, however, is not to. In this case, where it was done in error, there is no need to worry.
Still, since the Rebbe instructed on many occasions to people who erred in a Halachik matter that they should learn the halachos of that matter, you should, therefore, learn the laws of the nine days in Kitzur Shulchan Aruch. You should also give extra Tzedaka.