Hey Justin,
Unfortunately, I had the exact same experience at one of my first agency jobs. On my very first day I got thrown in deep and was told to fix an old site that was completely compromised by malware and who knows what sorts of viruses. I spent a good couple of days frantically trying to figure things out. Worst of all, the mentors who were supposed to be assigned to me just kept saying “hmm, that’s weird, I don’t really know what’s going on”… After a week or so, I thought I had it cleaned, so I handed it back to the client… and then just a few days later the virus came back again! 
Needless to say, that wasn’t the only case. I tried to explain many, many, MANY times that I was a front end developer (I had only just gotten my FCC cert) - but that didn’t stop my colleagues assigning backend work to me. It was seriously stressful and I basically had to learn AngularJS overnight, MYSQL, Node JS all over a weekend. I asked anyone and everyone for help, I asked a ton of n00b questions, I even looked on Upwork to find a mentor for a couple of weeks!
No matter what the task, you will have many moments in your working career where you simply need to bite the bullet and level up. Do whatever it takes. Invest in yourself, pay for help from mentors if you need to. I think there comes a time at all jobs where your manager pushes you to see where your strengths and weaknesses lie … I hope you can see that this test, while difficult to do and regardless if you fail, will definitely make you a better programmer in the long run.
My coworkers who were meant to be “mentors” completely hung me out to dry and unfortunately, the job didn’t last more than a few months. I just couldn’t keep up and felt completely overwhelmed - plus the support I was getting was frustrating because they would never give me more than a few minutes of their time.
Agency work is fast paced, that’s for sure! It’s a double edged sword because you do get exposed to a lot of cool stuff, but it definitely comes at a price. Spending late nights and weekends levelling up is exhausting.
Now the good news …
Yes I did learn a ton … and while I definitely did not retain everything I learned, simply cause I was under so much stress at the time, at least I could say that I had some experience on “commercial” full stack applications. I learned how to parse through thousands of lines of codes over hundreds of files, and debunk folder structures. I understood some basics with data management and environment variables. I learned AngularJS which was a gateway to Vue and React. I did templating with PHP and WordPress and built dynamic websites. After that job I freshened up my resume, and got back out there.
I hope you can find some good out of what you’re doing - the best thing to do is to journal your days so you can look back and explain what you did at your next job interview.
One year later and now I work for a software company and have a much more relaxed work environment where learning on the job is encouraged. Team mates look out for each other and we have bi-weekly standups as well as peer to peer code reviews which help so much.
I hope you can take away some good skills from your work, and I hope it doesn’t discourage you too much. If you burn out, then simply quit and move on. Don’t settle for learning some old tech or random programming language at company X if it’s not what makes you happy, and especially if you’re coworkers aren’t there to help - regardless of the salary! Be sure in interviews to ask hard questions about this 
Awesome question btw!