Poor puppy! As the owner of a very panic-prone dog, I sympathize.
One of the hardest things I had to get Meg over was her reaction to the electric fence at our barn. She refused to cross a particular barrier on the way out to the fields after she touched the fence. What ended up working was turning it into a clicker game (Meg was already clicker trained, and very interested in it.
What I ended up doing was going towards the limit of her comfort (so for your dog, maybe just to the inside of your door?). I would stand just a bit closer to the "danger zone" than she was, and as soon as she even leaned in the direction I wanted her to go in, I would click her and toss the treat backwards (away from the scary). So she basically got two rewards for being brave - the treat, and I was releasing her and letting her move away from what was scary. That also set her up to drive back towards me and the scary spot to get her next click/treat/run back. It only took a couple of clicks before she 'got' the game, and started to drive back a little further.
It did take several sessions, but she now will happily go past the scary area into her favorite hunting/hiking/swimming areas beyond it. I think it worked for us because there was no forcing (if I tried to drag her, she just got more scared and shut down), no luring (she wouldn't step forward over the line for steak), and no reassurance (which just seems to reinforce her fears). I'm not sure if it would work for your dog, but as I said, it worked for us.
Good luck to both of you.