jupyter notebook connection could not be established - multiple issues

I am running jupyter 6.0.3, which I just installed yesterday, 2020-05-26. I am on Ubuntu 18.04 and this is purely a local setup. This onslaught of errors started yesterday. A lot of the errors and solutions I have read of a similar nature seem to have stopped a year ago, but none of that helps me.

Yesterday, 2020-05-26, I got this error for the first time:
IOPub data rate exceeded.

I read of this issue and solutions here:
https://github.com/jupyter/notebook/pull/2293
https://github.com/jupyter/notebook/issues/2287
iopub rate limits are too low by default, for visualization-heavy projects #2287

launch with:
jupyter notebook --NotebookApp.iopub_data_rate_limit=10000000000

None of this made sense to me, as to why I should be having this issue, since although there was an import to matplotlib in my code, I had not gotten to the visualization part. Up to now everything I had been doing in the notebook was text based nlp. Nevertheless, it was possible I had an error in my code that was running in an infinite loop, so I tried it.

Now, however, Firefox was unable to open a new tab in the notebook, and my memory and swap were both over 90%. I also discovered that I was running jupyter version 5.2.0-3edaba4, so I upgraded to jupyter 6.0.3 and went to bed.

Today, the first thing I got when opening jupyter was a 500 internal server error. Sifting through the extensjve traceback, I found:
AttributeError: module 'tornado.web' has no attribute 'asynchronous'

The solution to this, which is from a year ago, was to install and set my tornado to 5.1.1, which I did. Only later did I read that this incompatibility with Tornado 6 was supposedly fixed in jupyter 5,7.

I saw something about websockets, and looked in the FF console, where I see this:
Firefox can’t establish a connection to the server at ws://localhost:8888/api/kernels...

Now my traceback comes to this:
File "/usr/local/lib/python3.6/asyncio/base_events.py", line 408, in run_forever
raise RuntimeError('This event loop is already running')
RuntimeError: This event loop is already running
Note: Jupyter itself says I don't have anything running.

So I shut it down and started again. Same Runtime Error. I moved to Chrome. Same error.
I've grown tired of chasing my tail. I don't know what to do, or undo. I am on Python 3.6.3, btw.

Any help would be greatly appreciated.

https://github.com/jupyter/notebook/issues/4630
https://github.com/jupyter/notebook/issues/3910
https://github.com/jupyter/notebook/issues/2664