Upgrade to Mountain Lion - macbook now freezes and cannot back up

I have upgraded from OSX 10.6.8 to OSX 10.8.5. Now my macbook freezes continually and time machine is struggling with a huge back which it seems unable to complete. I have tried backing up several times in the last few days but it wants to do at 21.5GB back up and at the moment has reached 750MB after an hour. I need help please with really simple instructions as I know very little about computers. Thank you.