I feel your frustration! I got Sandie 6 weeks ago (ok well almost 7 at this point I guess!) and at first, she was fine, NOTHING bothered her, not even doors slamming in the wind! Well that all stopped last weekend...she went past a door that normally doesn't bother her at all, and she BOLTED! About 4 steps into her bolt, she threw up a buck that caused me to fly over her head, do a complete flip, and land (thankfully) on my back!
Then yesterday I was riding again and suddenly one corner of the arena (that again, NEVER bugged her before) was scary and she bolted again! (but at least didn't buck this time so I didn't come off)...I'm starting to get really frustrated with the whole situation since it was going so well and now all of a sudden the strangest things scare her into bolting off!
It's really frustrating and disheartening but I talked to my trainer and he said to just keep taking her back to whatever made her spook, and while she's facing it, praise her and tell her it's ok. DON'T restrict the head though, let your horse move his head around and take in the whole thing, because sometimes when objects pass from their binocular vision field into monocular (one side of the head), like when you're coming up on and passing something, it's thought that the object appears to "jump" and can cause the horse to spook. So if you let him move his head and view it both ways, he'll be less afraid I guess. (according to the book "How to think like a horse" by Cherry Hill anyway! GREAT book)
Anyway, just keep working with him to desensitize him to things...that's what I'm going to be doing with my own horse...let me know how things go, and good luck to you both!!! Things will get better, you just have to give it some time!