I know how you feel! I had to give myself a kick up the arse this term because finalist year work is more bulky than ever. It does sound like stress rather than depression, because you're still happy doing other activities, and it seems to be just the workload that is getting you down.
So what I suggest worked for me - write a list of all the things you have to do (painful, but it'll get you organised!) and divide the work into little chunks. Get your dissertation plan ready and do a set amount a week, or a little task for it every day. This will release you from the negative cycle of not doing any work, then feeling stressed, then not being able to sleep...and it just repeats itself. You know you're capable of doing lots of work to get this far in your degree, so keep at it!