Newbe's cider

Have a batch of cider seven weeks into the process. Had no hydrometer. Five gallons cider, two pounds red organic grapes smashed, three Camden tablets, wait a day, five pounds sugar, packet Red Star Pasteur Blanc (proofed), bubbled real nice, 65 to 70 degrees, racked off yesterday into carboy, bitter taste, seams potent! Have space in carboy, maybe half gallon. Now have fresh cider can add. How back sweeten? Help!

Around here backsweetening is generally accomplished by adding enough FAJC(Frozen Apple Juice Concentrate) to top off the carboy after racking.
Buy some FAJC, let it come to room temp and dump into the carboy..no water or anything like that..just dump it in. Pop on an airlock and set it aside and let it clear.
Easy Peasy