I don't know why it took you so long as it came off pretty easily for me. Each door took 2 minutes or less. I got the top peeling (easy to do) and then just pulled with a grip on each side slowly downward. The first one ripped a couple of times, but the next three all came off in one piece. Now it looks MUCH better. After first reading this thread, that matte black tape on my black car was really starting to bug me. Plus, my buddy came over the other night in his black BMW with no tape on the pillars and after seeing that, it sealed the deal for me.
One more thing, after removing the tape, it does leave some sticky residue. The other day before removing the Sonic and LTZ lettering, I picked up can of Turtle Wax Label & Sticker Remover to clean up the residue left behind. It worked great to remove the sticker goo left behind from the black tape, too.