Answer:
The answer to this question: What does "this oath even forbade me" mean, would be: it meant that Abraham Lincoln, who was the one who issued this particular sentence on a letter, was incapable and powerless to break the oath to uphold the Constitution of the United States on the matter of slavery, even if he himself was naturally, and on principle, against that institution. However, he could not, as President, dissolve an institution that was protected by the Constitution itself.
Step-by-step explanation:
This particular sentence belongs to a letter that was written by the then American President Abraham Lincoln. This letter is known as Letter to a Citizen of Kentucky and it was written by Lincoln in 1864. The letter is in essence a long explanation as to why as President Lincoln will not take further hand, or do anything, to change the matter of slavery, despite his being against it. He insists to this citizen that slavery is protected by the U.S Constitution and therefore, he cannot, as President go against it, as he took an oath that literally stopped him from doing what he felt was morally right.