After reading that letter in its entirety, I do not agree with your assessment that Lincoln was “perfectly willing” to leaves slaves in bondage. The last paragraph seems to refute that fantasy.
Stop lying to yourself. You certainly won't get away with lying to me.
Lincoln never made any bones about his personal opinion on the topic. Yes, he was personally against slavery. We all get that.
That is all the last sentence says. It is simply a reiteration of his personal opinion on the subject. It does not, however, refute his very clear statement that he would not impose his personal feelings on the rest of the country. It does not refute his very clear statement that he would continue allowing slavery if it was necessary to "preserve the union." He said this 1 1/2 years after the war had started.
Any interpretation that Lincoln would not consent to the continuation of slavery is simply a lie and a distortion of his clearly enunciated words.
Lincoln would have continued slavery (at this point) if the South had stopped fighting and submitted to rule from Washington.
It is an unpleasant truth (from your perspective) but I urge you to accept it and deal with it.