Previously you noted that the snake "seems very friendly", but after the introduction of the mouse she seems snappy? I am not there, so obviously I don't have much information, but is it possible she was snapping at the mouse? Was it live or frozen/thawed?
You really shouldn't feel bad for trying. Many people, myself included, have had snakes eat the day after arrival without issue. It is perhaps more typical for them to feed after a week or so, but by no means is it always the case. Also, as you have no doubt discovered, heat lamps are the bane of humidity. A UTH will make it much easier to regulate and lots of keepers use two thermometers like yours are set up (one on the cool side and one on the warm side), so you are hardly silly.