you're definitely NOT losing slower than everyone else. When I was where you are (four months out), I'd lost 52 lbs. And I started out much heavier than you.
people lose at all different rates for all different reasons (age, sex, metabolic rate, activity level, whether or not you lost weight prior to surgery, starting BMI, for starters...). Other than sticking to your program and your activity level, you don't have a lot of control over these factors. Ultimately, your success is determined by how committed you are to your program - NOT to your rate of weight loss. I think losing 100 lbs in six months isn't that common, to be honest, except for people who start out much heavier than you. It took me a year to lose that much, and again, I started out much heavier than you did.
I was a slow loser the entire time, and I, too, was told by some insensitive medical resident that I was behind where I should have been at every juncture. But I ended up losing 100% of my excess weight - which means I probably blew most/all of the other people in my classes out of the water by the time I was done.
so just keep up your commitment, follow the rules, and you should be fine.
P.S. yes weight loss does slow down after the first six months or so, but i kept losing until month 20. So don't give up hope!