I would start by taking things in small steps. For example, start by working to declutter/assess/toss stuff out from just one room in your house. When that is done, move on to the next room. If even this is overwhelming, pick one side of a room to start with, and work your way around the room. Don't let yourself get distracted by shiny objects in other parts of your home - just focus on that one area until it is done.
I have found that breaking things down like this makes it so much easier to work through a large-scale project. I struggle not to attack everything as I see it, which creates for a very scattered approach, but I've learned that mentally I'm better off just focusing on one discrete area at a time.
Take it slow like others suggested, and start in one room at a time, or even one piece of furniture/one drawer at a time. Separate things out into KEEP, DONATE, SELL (if you are interested in doing a yard sale), RECYCLE, and TOSS. Throw away the toss pile then and there. Same with the recycle stuff (papers, magazines, old mail that doesn't need to be shredded).
I would recommend you purge BEFORE you need to start packing. It will give you a chance to lay eyes on everything you own in slow time, with no pressure to make a decision because things have to be packed right now! When packing, there is a strong temptation to just dump everything in a box and deal with it later if it gets overwhelming.
But when you purge, keep in mind that you are doing this BECAUSE OF THE MOVE. Ask yourself, with every item, "Do I need this/want this enough to spend money on moving it all the way to the new house?" If it's something you haven't even laid eyes on for years or that you completely forgot you even owned, then the answer is probably NO.
I did ok, I got rid of a bunch of stuff via donation and yard sale.
There's still so much. We're in the new place and I'm being really critical and realistic about where I want things and what the limits are on our storage areas.
And new rules, like no particle board anything and no cheapy plastic crap. Anything gross is gone, anything new is quality. So that what I do get is good stuff.