I can relate! My old (first) scale underestimated my weight by almost a stone.
I realised it was out when I weighed myself at my parents and my grandparents homes. Two scales couldn't be wrong!
So I got a new scale and started over with that. However, I also kept the old scale for comparison purposes.
If you know your starting weight on the wrong scale, you can still count how much you've lost by weighing yourself on it occasionally.
Ok, so you may weigh more than you think, but you have still lost an impressive amount of weight and you can continue to do so. Don't let it dampen your spirits, you can do this