Have a cup of tea, remind yourself that whole bin bags of stuff have gone and the house/world is still standing 
Then make a start on one area/room to clean until it is totally decluttered and shines. I use the bathroom as my motivation (it is less scary) ... old half bottles of stuff that hasn't been used forever, bin it. Re arrange it, tidy it until everything has a place, then clean it all until it squeaks.
Then, maybe tomorrow, use that success to tackle every single drawer, bag, box, nook and cranny in your worst room. I have posted this a few times, it always works for me:
- Take 3 bags/boxes: throw it, keep it, give it away (you will need refills)
- Start in one corner of a room and put EVERYTHING into one of the bags/boxes.
- Throw away the rubbish bags immediately
- Take the give it aways to charity or recycling immediately
- Clean the room until it is as clean as a very, very clean thing.
- Slowly put everything in the keep bag/boxes into its place. If it does not have a place rethink it. If you want it but don't have a place for it put it in a MIGHT BE bag/box
- Do the same for every room
- Open the MIGHT BE bag/box and make a final decision on every item.
- Have a cup of tea
10. Tell the rest of your family how to keep their space clean and tidy...
If I do it all in one go it takes me a week to do the whole house, loft included. I try to do a room at a time, to save me feeling too overwhelmed.