My Spanish is intermediate and in the rare case when I was teased about my yankee accent, it was all in good fun ... Actually just yesterday in una tienda I was complimented on my pronunciation! Happens more often than not that my Spanish is complimented, because locals know, at the least, I am trying and learning. In my case I have found Porteños on the whole to be very warm and welcoming. Maybe, as 'outsiders' we need to buck up and not take offense so easily. And not to take ourselves so seriously perhaps. All part of the challenge and adventure, in my view ...
And what's even more funny is when I have 'massacred' my intended comments in a few group settings - For example, taxi driver says, 'how's it going'. My yankee reply was intended to be, 'oh, no pudo quejar'. Instead I said ,'oh, no puedo cagar'. The laughs continued for the whole ride (including at myself). Greatest hit #2 - Greg, cuantos años tiene? I responded, 'tengo 55 anos' at a table of 15 with an uproar of laughter! Great fun ... I have no problem with my errors and, as I mentioned, folks know I am trying and are very good natured about it.
Best advise - Relax, keep practicing and it comes. Little by little ...