Retrospective help

[epalumbo]

Hi all,
Sorry for the first long post, and I appreciate any insight I can find here.
I recently lost my first pet snake after only a couple days and I'm at a complete loss as to what I could've done wrong. I'm not entirely sure what I'm looking for at this point, honestly I'm afraid to try again for fear of losing another animal, but I figure maybe if I can get some light on what happened to me, someone else might have some better luck down the road.

I recently received a baby anery corn snake. I had set it up in its enclosure, which was a 40-gallon terrarium. Substrate is cypress mulch with some spaghnum moss on top for cover, a 100w heat lamp on a dimming thermostat, a heat pad (mostly for at night or if the bulb went out, its temperature was always set lower as a kind of backup and it never really turned on except for towards the middle of the night), a UV lamp, several hides and climbing branches, a shallow water dish, a hide with lightly moist spaghnum moss as a humid hide, and a couple small pothos plants in pots as decor.
Temperatures during the day stayed at 90º in a basking area, and the rest of the tank stayed 80ish on the warm side and 70-75 on the cool side. At night, the warm side was around 70º. Humidity stayed pretty constantly at 60%. I double-checked temperatures with a temp gun about 10x a day just to confirm it was all in spec.
When I first got it in and introduced it to the enclosure, it warmed up for a few mins, and then took off exploring the enclosure. Pretty quickly it found the humid hide and the hide on the warm side of the tank and for a few days just alternated between these. For a few days I did nothing with it other than occasionally move it to make small adjustments to the enclosure, including adding the spaghnum moss when I thought he might be concerned about the lack of cover in the enclosure.
A few days later I fed it a whole pinkie, which it took really quickly. It took a good 10 mins to get it down but it made it all the way to the stomach. I left it alone for just over 24 hours, just peeking in to make sure it was all going OK.
I found the (I assume regurgitated) pinkie that morning; it was still largely intact and smelled kinda bad. I had at first thought it was poop because it looked pretty digested but it almost surely wasn't digested. The snake seemed to be acting fine, climbing around on top of the smaller branches and generally looking healthy.
Within a few hours, it was acting more erratic; moving with a sideways tilt, doubling back and forth on itself, kinda acting disoriented. I called the store I ordered it from to ask advice and by the time I returned to get a video of what it was doing, it had stopped moving pretty much at all. It didn't react much to me, no tongue flicking, no looking around. I picked it up and moved it towards the middle of the enclosure as it was on the cool side and I was afraid it wouldn't be able to get around to regulate its temperature in this state, and it acknowledged I picked it up but didn't exhibit the usual curious behavior I had been seeing. Really slow tongue flicking, and didn't want to move its head. Just to clarify, the time between seeing it alert and exploring and finding it barely able to move was maybe 3 or 4 hours, I barely had time to react to the change.
As I type this, the poor thing is still breathing and that's about it, but the shop can't figure out anything else to help and I have no resources to fall back on to try to help it out any more. All I'm making sure of is that it's at a comfortable temperature because there's nothing else I can figure to do.

What could've gone so wrong in the ~week I've had it?

Thanks for reading this far, and for any help you can render me in understanding what happened. I'd love to try again but I can't in good conscience do that without at least knowing that it was something out of my control or that whatever I had done wrong was remedied.