Darkness or Light? Philosophy in Knights of the Old Republic.

See, I told you that all light-side Jedi who turn to the dark side lose their power... Well, or Obi-Wan was just stronger than his apprentice (which is quite likely). Maybe Anakin is physically stronger than Obi-Wan, but in terms of wisdom and using the Force, he still has a long way to go...

Using the Force doesn't always depend on wisdom... In terms of using the Force, they were equal - this is well shown in the telekinetic confrontation when they both flew in different directions... the stronger one would have just stayed standing...
Anakin just uncontrollably unleashed his emotions, his rage... Anger can add strength, but it can also make you overconfident and clumsy... and Obi-Wan took advantage of that... In general, the duel was silly... they were like pumped-up fencers, only fighting with lightsabers, but rage directed at using the Force could have clearly helped Anakin more... which is what Palpatine did in the battle with Yoda...
Anakin was a beginner and therefore hadn't yet learned the secrets of Sith power... unlike Palpatine, who was more cold-blooded and calculating, which gave him an advantage over Yoda...