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? 
I'm sorry you have such a shitty coworker. They have no excuse.
Ask them to not do that in a way that is polite and centers that it makes you uncomfortable, in an email. Given their behavior, I think it is unlikely this will work, but if you do this both in-person and in an email, you will have documentation of attempting to resolve this in the corporate-friendly way, i.e. "amongst yourselves". It is okay to skip this step if you find it seriously detrimental, but it will make any other steps a bit less likely to go smoothly.
Then you'll want to go to your manager. This is the most critical person in the company for a positive resolution. If they don't care or don't take action, or even turn in you as a "troublemaker" for making them deal with this, then unless you plan on getting a new manager (e.g. it is a large company and you can transfer or your boss gets fired) then HR will not be useful to you in any way. They will just try to cover the company's ass legally and the only way they do that favorably is by going after your boss. So I recommend thinking of this as 3 tiers of action.
Tier 1: ask the person to stop, politely (even though they don't deserve this), in an email stating that it makes you uncomfortable.
Tier 2: go to your manager with this problem. State how it is making you uncomfortable and that they are not changing their behavior.
Tier 3: go to HR if and only if you think you can get your manager fired or if they can do an in-company transfer. Getting your manager fired would itself take a battle that would be draining, so while it is a valid thing to do, it is also okay to focus on your own well-being over taking the fight that far.
A secret hidden tier: as an alternative to talking to HR, just don't. Document all of the issues in writing, dating them. Ensure that your polite and clear requests have an email version, particularly if anyone is dragging their feet (email to coworker, email to boss). Back up that email + take screenshots. Keep a written and backed up record of anything that could remotely be interpreted as retaliation. Just keeping a dated notebook that describes what happens each day will keep you covered and can also be therapeutic imo. This secret tier is all leverage for keeping your job while you look for another one and fuel for a lawsuit should that become an option.
Unfortunately a lot of work environments are incredibly toxic and make us choose between staying a fighting carefully like I've described or just leaving to find something better. It sucks, but starting the "find something better" process can be helpful in itself as you will eventually get an offer and can feel less beholden to any and all assholes involved.
Edit: do Tier 2 at the same time as Tier 1 so that your coworker can't innoculate your manager against you with lies.