Bascially Anakin was in a very unstable mental condition which rendered his clarity and perception making him less combatively powerful during the fight. In addition to all this, Obi Wan knew Anakin and his fighting style really well to the point that he could instinctively react to Anakin simply based on this. If Obi Wan wasn't Anakin's master for years, he would get stomped just like Dooku.
Although I don't think Obi Wan being the best defensive fighter was the biggest factor in his ability to hold off Anakin. Obi Wan got slapped by Dooku despite this. The biggest factor that allowed Obi Wan to last against Anakin was his knowledge of Anakin and how Anakin fights allowing Obi Wan to react instinctively. THis and Anakin's mental state were the biggest factors.
Originally posted by qwertyuiop1998
1 Anakin is overconfident and his mental condition is very unstable( As we see him force-chocking Padme whom he wants to save, His mind is a mess during this fight)
2 Obi-Wan knowing Anakin's fighting style too well
3 Obi-Wan is one of the most defensive fighters in SW
Really well summed up 👆