Jamieson, Fausset and Brown on Romans 7:14

14. For we know that the law is spiritual--in its demands.

but I am carnal--fleshly (see on Ro 7:5), and as such, incapable of yielding spiritual obedience.

sold under sin--enslaved to it. The "I" here, though of course not the regenerate, is neither the unregenerate, but the sinful principle of the renewed man, as is expressly stated in Ro 7:18.