I broke down today in a public setting when someone deliberately attempted to annoy me because they enjoy my reactions when I get upset. My question is, how to deal with such things? Any hexbears who regularly deal with this or have suggestions on how to handle such situations? Am I just fucked with dealing with assholes?
Content Warning: self-harm
So, I get sensory overload pretty easily with seemingly small auditory cues, such as whistling, intermittent humming, low frequency vibrations (like from old AC units or fluorescent light bulbs). Someone I am forced to interact with on a daily basis decided they wanted to make me squirm today by whistling off-key repeatedly, loudly, and very near to where I was working. When I asked them to stop they continued to do it, kind of like a sibling who is bored and wanting to get some entertainment by driving the other party crazy. After 20 minutes of it I was getting to the point of distress, and I asked them to please stop because it was making me uncomfortable, and their response was to try and do it more loudly. I finally went to them and talked to them directly, face to face, and all but begged them to stop because it was making me uncomfortable and it was getting disrespectful, near tears at that point, and they rolled their eyes and said "Sorry you got triggered". The reality was that I was to the point of starting to scratch myself with my nails to distract myself because I was so distraught from the noise, something that I've come to understand happens when I'm starting to dissociate. I'm ashamed to say I dug my nails in enough to draw blood and leave marks, something I haven't done in a long while. Ended up leaking a few tears, which is really embarrassing and shameful for me when I'm not alone...
I have to work with this person every day, and I can't wear headphones/ play music/ do things to block them out like I normally would. I just got this job but I'm already to the point where my mental health is being trashed after just a few weeks.
Am I SOL? Anyone have anything that could help with this, even if it's suggestions on dealing with over stimulation in a work setting as someone with autism? I keep my ND a secret; no one knows I'm on the spectrum. ...am I overreacting? 
Not overreacting at all this person sounds like a malevolent piece of shit. Unfortunately I don't have any good advice I've never had a positive resolution to workplace harassment and offices are such a hellworld for my autism that pre covid was a perpetual downward spiral where I completely depersonalised and self harmed and NT people thought i was crazy/enjoyed making it worse to see me uncomfortable.
Maybe a neutral approach would be to ask your supervisor permission to use headphones if there's a way to be alerted to a call via some other visual or sensory means and give the reason "the office is very noisy which I'm finding distracting which is harming my productivity i believe if i can use headphones my productivity will improve" maybe even ask for a trial period with it to prove you can operate with headphones.
Usually to get some amenity i have to phrase it in a way that makes it sound good for my output. Other than that I know exactly how you feel and I'm sorry you're in that situation.
Thank you, I've worked in offices a lot and it's always been as you mentioned, hellish and demoralizing. Haven't been in an office since before covid either and it feels like people are even worse than before. I'll try some of the things your and others mentioned, see if I can't give myself some relief and support.