If you are about to move into an assisted living facility, be it an independent senior community or a nursing home, you're going to have to let go of a lot of stuff. Unless you were already living in a small space, chances are you've got rooms full of things that just won't fit into your new living space. It might not be easy to downsize that much so quickly, but it can be done.
Remember That Storage Is an Option
First, you may want to arrange for much of your stuff to be placed in a storage unit near the assisted living facility. This gives you more time to go through everything and make sure that you're keeping only those things that you truly want with you. If the financial arrangements with the assisted living facility call for you to turn over most of your assets -- many facilities have policies like this -- see if a family member would be willing to spring for the cost for a couple of months, or prepay for several months of storage before turning over the rest of the money to the facility.
Most Furniture Can Go Immediately
Chances are that most of your furniture can be donated immediately. If you're moving to a senior community in which your space will be unfurnished, you might be able to take your bed and some basic furniture with you, but most of what you have can go to a charity quickly. Many will help arrange pickups, or you can have a local "small-move" moving service take the pieces away. Again, if you didn't have much to begin with, this might not apply to you. But for those who have had well-furnished large homes for years, there's likely to be a lot of furniture to sort through. Get rid of most of it through donation.
One of the reasons people like to hold onto things is because those things represent past memories. However, if you have to downsize quickly, you might not be able to keep all those books, paintings, and knick knacks. If you have things that you keep mainly for the memories, even if you don't read them, use them, or even really look at them, take pictures of them. Capture all angles and make sure the photos are clear. Sometimes the photos are really all you need to bring up those memories again.
The facility you're moving into will be able to give you an idea of how large your space will be and what sort of items will already be provided. Start sorting through your belongings as soon as possible, too, because the more that you can get out of your place before the move, the calmer you'll feel about the whole thing.