Your current weight loss is on par with standards for losing healthfully, so way to go! However, goals are rarely achievable if they're not S.M.A.R.T. That is, Specific, Measurable, Attainable, Realistic, and Time Bound. As others have said, weight loss tends to slow down at a certain point and there's also the possibility of plateauing - don't let this discourage you. Instead of looking at December as your end date for your total weight loss, why don't you get yourself a goal of 3 months and select an amount of weight to loose within the next three months that is suitable to you?
If you change nothing, nothing will change.