I have never heard before that Forrest was an abolitionist. Do you have a cite for that? I'd love to look into that. I know late in life he reconciled with the defeat and urged racial healing.
He freed the slaves that he owned that accompanied him to the war in 1863. (see Nathan Bedford Forrest's Escort and Staff, Michael R. Bradley)
Just before the close of the war he manumitted his remaining slaves. (George W. Cable wrote out the manumission papers. He documented this in letters to his mother (presently housed at Tulane) and also in an essay after the war.)
Like Lee and Cleburne, Forrest advocated recruitment of slaves into the Confederate army, however, he also proposed that they be given freedom and promised land after the war. (This appears in period newspapers as part of the debate on arming slaves.)
He advocated passage of the 13th amendment. This can be found in his Congressional testimony. It is also mentioned in papers of Isham G. Harris, and James Alcorn.
He strongly denounced slavery after the war and offered he would fight any man who tried to bring it back. He declared it a failed system.