It is worth keeping the appointment, do you have someone who can go with you?
It isn't particularly fun, we had an assessor who didn't cover most of the questions he needed to, they also, irritatingly, will assume that the state you are in at the assessment is the worst state you are usually in. (Ignoring the fact that with mental health problems you'll have had to psyche yourself up to go)
So do go when you are feeling really awful, shit as it is, they need to see it.
I'd recommend having a look at how points are awarded too, so that you can try and remember all the the problems you have that are relevant. Points here
If you can get someone to go with you, show them as well, so they can remind you if you forget.
They are fairly crap at dealing with mental health issues, so you may be turned down, but you will have a chance to appeal.
For this it is probably worth getting a note from your doctor. I would also talk to Mind about getting their help.
Really hope it's not too bad for you, and that you are doing ok at the moment.