Things are not going well, I'm way behind the schedule for the work. It feels just like the corporate world, only I had no control or input over the project details or remit: all was done before I arrived. As someone who is very self motivated and driven, this really upset me. I felt I deserved to have more control over my career rather than cede all responsibly to be a cheap outsourced "research unit".
Deep breath! It's hard to see clearly when your project is in the weeds.
There's probably a reason that you had "no control or input" over the project details. Is your pay coming from a grant to the PI that the PI wrote prior to hiring you? If so, then you must work on the project that is providing your funding.
I think, though, that you have a wonderful opportunity here. The project your PI proposed isn't working, right? It's time to make it yours. While you
do have to address the research goals stated in the grant, you can do it your own way, and it sounds like you need to do that. Read that grant application backwards and forwards. Read all the references. Can you find a better way? Go for it, postdoccer!