I'm really sorry to hear this OP
I'm in a new build that's riddled with issues.
The worst of it is..that's not the worst of it! I bitterly regret moving and had a chance to pull out of the sale of my old place but didn't take it for reasons too complex to go into.
I am now 8 months in, still crying for my old home, and considering re-buying in that block if the chance comes up.
You mention not knowing how you got it so wrong. In y case, one driving factor was my elderly mum, but now I can't believe I let that influence me over something as fundamental as my home. I feel utterly lost and hopeless and dread the future.
It's such a horrible situation but tends to elicit zero sympathy as it's so first world. But I really get it. 💐