I wonder why the "U-turn" happens?
I think you must have a negative "groove" in your brain which is there because of bad experiences. This is very common but it can be changed. Our minds are not set in stone and you can re-wire them to be positive.
My mother is exactly like this. Say we plan something good and I ask her if she's excited she says no she's worried about all the things that can go wrong! Her brain has a negative groove so deep she's pretty much stuck with it (she's 73). She doesn't have to be like that but she can't see a way of changing it. Don't be like her lol.
I haven't read all of Im-Suffering's reply but I'm sure there's some good advice there because this is his specialty
All the best,
Gypsy x