Good job on the loss so, you should be proud! And good luck on the rest.

Come join us in the 20-somethings thread if you haven't already. There's a thread in there for others who are trying to get out of the 280s.
As for the motivation to exercise, perhaps you could try planning out a workout schedule for the week/month, and by the end of the week/month, if you've stuck to your plan, you could reward yourself somehow? Just something small like a new book or shirt or something that could help keep you going even when you don't feel motivated. If you just work to make it a habit, it will get easier to do over time.