@wildtoast , my top tip would be to accept that T1 has an inherent unpredictability about it, so it's a case of accepting that doing the same thing two days in a row can result in different outcomes for no apparent reason.
While it's attractive to think that 1 unit of insulin will always deal with 10 g of carbs or whatever, these things aren't set in stone. If it turns out that the same u I took for the same meal isn't playing out like it did yesterday, it doesn't mean I've got it wrong, it just means that the interplay between the insulin, food and a million and one bodily processes which they are subject to are playing out differently than yesterday, so the treatment for that meal will require an after the event adjustment of a little more sugar or insulin to bring it into line.
It's a moving target, so the trick is to be nimble and move with it. One brilliant tool for doing that is cgm, I second all the posters who have recommended it.
It levels the playing field so much being able to see how bg is moving, letting you make subtle adjustments - 5g here, 1 or 2u there - to keep things gently in line instead of taking a sledgehammer to it if it's gone way out of range. Plus it'll wake you up if you're heading for a hypo while sleeping!
And you can kid on you're a stockbroker - I've actually had a couple of shoulder-surfers in bars seeing the graph when I've been standing at the bar asking me how my stocks were doing....
There's a couple of good books on the topic - Sugar Surfing by Stephen Ponder, and Beyond Fingersticks by William Lee Dubois.
As a pharmacist, there's a couple of developments going on which might interest you from a professional, and now personal, point of view.
"Smart" or glucose responsive insulin: inject whatever amount and it switches on and off in response to glucose concentration, so you wouldn't need to bother calculating doses.
Also, HDV, hepatic directed vesicles, it's added to normal insulin to 'taxi" it direct to the liver to tell it to suck up excess glucose. In non-T1s, the vast majority of insulin goes straight from the pancreas to the liver to deliver that instruction, whereas our injected insulin rarely gets anywhere near the liver. HDV is meant to change that.
These are both at very early research stages, so who knows what will become of them, but worth keeping an eye on.
And if it all gets too much, just remember Eva Saxl, an inspirational woman! She and her husband Victor, were Jewish WWII exiles in Shanghai, insulin supplies were cut off after the Japanese invasion. Did she curl up and die? Heck, no, they found some technical papers on how to make insulin, set up a makeshift lab, got supplies of water buffalo pancreata from a friendly butcher and made their own. During a war. Saved her and several hundred others. Whenever I feel narked about my T1 throwing a wobbly, I just remind myself I've got it easy compared to Eva. Google her, there's an interesting short video.
Good luck, it's a head-spin to start with, but you will so manage this!