I really believe that most clutter is the result of fear. From fear that we'll be caught short to fear that someone will be mad at us if we get rid of something. So you need to try to relax.
Promise yourself forgiveness if you get rid of something today and then figure out a use for it tomorrow.
Learn the difference between needs and desires. And then learn to be comfortable with your unfulfilled desires.
Take a trip to a second-hand store. Go to the auction. Be sure to leave your cash and credit cards at home! Just look at all the stuff on offer. Whatever you get rid of today can be found there, cheaply, tomorrow. It's OK to let go. You can relax.
Go for a good long walk to clear your head. Then go home and fill another box with unnecessaries. Think of it as buying yourself some breathing room.