High Sierra really, really, really messed me up

To be honest, and I'm sure you know this really from what you've said, this is a problem you've made for yourself.

First, I can't stress enough - ALWAYS have a backup before you make major changes - an OS change is a huge change. Coming from the world of Windows, Apple couldn't really make backing up & restoring a Mac much easier so there's really no good excuse - it's a really simple thing to do and costs no more than a cheap USB Drive. That would have been the obvious escape route for your issue.

Secondly, if you had "work" to do and needed to do it relatively urgently, perhaps choosing to upgrade your OS wasn't such a clever idea - and would have been better postponed until you had some free time to account for any issues.

Obviously you know these things and it's all hindsight, but every day I see people just like you that could have taken really simple precautions to prevent the problem being such a big deal. Hopefully it will be a lesson to you and others that just assume computers are magic and can't possibly go wrong...

I guess you need to consider reinstalling High Sierra again and hope it straightens itself out, or give Apple a call (assuming you have AppleCare ... although I imagine you don't for the same reasons you didn't have a backup etc...) and ask for some advice as it sounds very much like the OS upgrade did not complete properly at all given the list of things you have that have gone wrong.

It's good practice to be backing up your computer regularly. Apple provides TimeMachine, it encourages you when you get a Mac to use it - and if you're doing work on your computer, and you value your data it's a **** simple way of backing up and making sure you're good, not just for an OS upgrade but in case anything else happens.

The same "notification" you get to get the OS upgrade would previously have notified you (maybe a long time ago sure) to setup Time Machine - and had you done this and taken some basic steps you'd not be in the mess you're in now, and the problem is that the recovery options for fixing your problem start with the ability to use the backup you don't have.

It's not a case of talking down to you, it's a case of you've posted about a problem, and the most common ways to get you up and going again you've eliminated by your own admission.

Unfortunately whilst people might not have a "special spidey-sense", it'd be good if people paid some attention to the basics of having a computer - and Apple goes to quite a lot of effort to make it easy to backup, and to notify you of the idea of backing up - I mean heck you setup TimeMachine - it prompts you, it takes seconds, if you forget to because the drive isn't connect, it reminds you regularly (the same notifications for the OS upgrade - so you obviously see them), and it's pretty hard to see how you could not think to take a backup. It's a **** sight better than the half baked backup in Windows, and I do find it pretty amazing that Mac users can't avoid the pitfall of not having a backup when Apple make it so **** easy.

Whilst we're at it - just because previously an upgrade might have gone OK doesn't mean it always would - I drove home without an issue yesterday, but it doesn't mean there won't be traffic today. The simple reality is that there are millions and millions of Macs out there, all running different combinations of software and setup, and whatever Apple might try, they're not going to get it right 100% of the time - nor does anyone else because computers are actually pretty complex.

As I say, in your case since you didn't take any of the sensible precautions (presumably because you were too excited about getting the "upgrade" (presumably you also didn't think to check if the software you rely on would be compatible either...)) you've made this situation messy for yourself to fix...

Also the High Sierra upgrade makes some massive changes to your Mac setup - and I do think Apple should make it clearer but this is a big ticket upgrade with a lot of risk.