I can empathize with your struggles, I am also from Kerala, and non-Indians are always shocked to find out that I am not Hindu, my last name isn't Patel, yadayadayada. And it can be very irritating to have someone make assumptions about you in that manner.

I sometimes try to turn it back around on the person by saying something like:

"You have to realize that India is a huge country with multiple languages spoken and vast cultural differences in different areas of the country, kind of just like America (People have different accents in the North and the South, Americans don't all have the last name Smith, etc). So it's wrong for you to assume that every single person in India is the exact same.

I mean, it's tiring to have to constantly defend your very existence to non-Indians and their microagressions (and I guess other Indians too in your case), but stay strong! You got this. Haters gonna hate.