253. thanakkAgak koNda sArathya vEshaththai avanaiyittup pArAdhE thannaiyittup pArththu anjina accham theera, thAnAna thanmaiyai ahamenRu kAttugiRAn.
Sri PBA Swami's Sarartha Deepikai:
Previously, in the 'mAm', He had refered to Himself as the charioteer. Here, He is using 'aham' to show His supreme independent self.
He said 'mAm' refering to Himself in the garb of a charioteer, and asked that Arjuna surrender to Him. Seeing Him in this role, which He had taken to protect Arjuna, Arjuna forgot that He who is Supreme had done this for him. Instead of seeing this from His side, that 'the supreme protector has come down so low out of His own divine qualities', he saw it from his side, that "This person who works for me and is my charioteer, is asking me to give up all dharmas and surrender to him. If I do that, what would happen to me".
In order to remove that fear, He is showing His supreme nature and is refering to His unbridled independence as 'aham'.