Can you get observations from some of the players, or another goalie, after your pickup session? In the heat of play I often have no idea why I let a stoppable one in, but its stoppable status is usually visible to everyone.

I make it a point after the game (and this is easier in pickup) to seek out someone (preferably the guy who scored on me) to ask what I was showing that could have led to the goal.
Lots of times it's something fundamental that you're not even aware of. For me, having played a lot of goal without the aid of defense, I sometimes overanticipate that the puck will be passed to one of those wide-open players standing in a high-percentage scoring area. While it's good to have a read, overanticipation can cause you to lose squareness to the puck, which gives the guy with the puck more incentive to shoot.
Lifting my stick is another popular brainfart, as is leaning too far forward in my stance and not maintaining active hands. Here are some things I cycle through when I'm giving up ones I should stop; of course YMMV:
--Are you getting pieces of shots before they go in, or do you feel like you're just missing them? If so, check your positioning and make sure you're square to the puck.
--Also, if you find yourself not catching as much as you used to, batting at the puck or not being able to direct pucks with your blocker the way you want to, make sure you're maintaining active hands.
--When you go down, do you have trouble with upper-body balance? If so, make sure your torso is more upright in your stance.
--Stick down speaks for itself, but pay special attention when you are shuffling.
Visualizing is a good tool; I also find a quiet place where I can skate before play starts. It doesn't have to be too large. Get into your stance, do some drops (slowly at first) and make sure you are fundamentally solid. This will allow you to correct any flaws at your own pace in a controlled environment, then make additional tweaks during warmups. This will get you as ready as you can be once play starts.
Hope this helps; let us know how your first game goes.