It's sometimes hard to stay motivated once you begin the process. You spend hours on a box of papers, and think to yourself afterward--That's all I did? I hardly made a dent!
In order to stay motivated, you need to turn your attitude around. Instead of seeing what DIDN'T get done, celebrate what you DID get done.
Look at the pile you finished, and the garbage or donations you generated from the work you did. Maybe you only have one paper bag of papers to shred, or one small box of clothes to donate. Either way, that is one more bag or box than you had when you started.
If you feel you work better when someone is directing you, then hire a professional organizer. Or, if you just need someone to talk to, then have a friend over. The time will go more quickly if you turn the experience into something pleasant.
Also, the decisions will be easier as you progress. There will be more hesitation at first, and more emotional ties to your belongings. But once you start letting go of your things, you will find it easier and easier to place them in the donation pile.
Think of it as a cleansing of the old to make room for your new life.