holy crap - i can totally understand how you are feeling. jumping up 30lbs in one sitting would crush me too.
how about getting a proper reading at a gym or the doctors - at least you will know where you stand and which scale is correct. the difference between the two scales is huge!
you can do this. use 300 as a motivator. tell yourself that you are not going to allow yourself to even contemplate reaching that weight and you will start moving today to ensure that doesn't happen. use your fear as the motivation.
i know it is easy to sit down and go "whats the point?" but doing that is not going to help you lose weight. Here is a link to a thread I made ages ago that has some info about negative thinking and the defining moment for me to stop saying "what difference will it make?" or "i'm too fat and the ship has sailed for me" -
The Defining Moment
you can do this and we are here for you!