First and foremost, I dont care how you feel.
Secondly, I really hope you do feel miserable and humiliated just generally, even if not the result of my comment. The USA is precipitating unspeakable harm that will resonate around the globe for decades.
Finally, the term seppo isn't intended as an insult. Its a jab or a jeer. Like calling an American a yanks, or calling your friend hairy legs, or a queenslander a banana bender, or a western Australian a sand groper.
I wouldnt call my girlfriends father a seppo, but an American im on friendly terms with, absolutely.