Perhaps it's not the rejection, but the energy in creating the next steps I need to take (more resumes, taking classes, new career, therapy, etc).
Sounds extreme. Were you applying to random jobs?
For example: As a candidate you might submit a resume and get an automated "Thank you, we'll be in touch". There's a large variety of reasons why you might face a very low probability that someone will indeed be in touch that have nothing to do with you.
The most common is that you're just too late in the process. They might already have dozens of viable candidates in later stages. They are being economical about their interviewers' time, thinking: Well, the probability that all of those later-stage candidates will end up getting terminated is low, so we're not doing first-round interviews for now. But they are not being economical about expectations created in candidates' minds about the likelihood of getting an offer. They've simply defined the process to say: We pull the advertising after the role has been filled, because there is always the chance that we will have to go back to candidates in earlier rounds.
There might be even more extreme cases. Not saying that this is particularly common, but imagine a company has decided they need a diversity hire, say hiring a woman to an all-male tech team. All applications by men may be dead on arrival. They may not want to send out rejection letters so as not to establish the pattern.
Or the role may have already been filled and there may simply be clerical error or clerical laziness to blame about why the advertising hasn't been pulled yet.
So, from a candidate's point of view: "We will be in touch" means almost nothing. Having come across a job ad means less than nothing. Your calculation is simply going to be: I get one human contact out of every ten times I submit a resume. I get one firm offer out of every ten times I get a human contact. I'm shooting for three firm offers to choose from. So that means I have to submit my resume 300 times. Consequently you can't afford to be all too picky. You just submit to everything that could even remotely be a match. You only start putting real brain power into thinking about whether you actually want to work there in later stages of the process.
It's a sad state of affairs, but it's just the world we live in now.
Luckely my experience in job searching has not involved ghosting or spamming. Is that an US thing? Or is "job dating" less parallell in smaller cities or countries?
The whole point of web-based application process is to weed out as many candidates as possible. If you can skip that step and first make contact with a human, the odds are much better and it's less work and stress for everyone.
Tailoring resumes to specific postings, and trying to find someone at the company you have some sort of connection with, are both pretty damn important unless, as you say, you're shot-gunning an identical resume to every company within 50 miles.
I would recommend that you attend local user group meetings and talk to people. Weak social connections are an excellent source of referrals.
At the very basic level you should understand what are the expectations of the employer and whether you can fill those expectations. Try to find and talk to people at those positions you are applying to. Try to understand where you fall short.