Hi Bragi,
I worked in a design/manufacturing job for years and would often overlook small details in making an order, requiring it to be remade and for me to be called on the carpet by my supervisor. Only recently have I realized how anxiety fueled that dynamic- make an error, reprimanded by boss, anger/fear, harder to focus, more errors.
My only thought would be to take a teeny window of space before undertaking a given task, say, several deep breaths followed by physically writing out what you'll do in manageable chunks. This is the kind of thing I wish I'd done in my previous job. This kind of thing may help to amp down any fear you're carrying at a given time so that you can function more effectively.
BTW, thanks for the response re: follow through. I've tended to stop projects not becuase they're no longer exciting but because of fear of failure. Still working on that one, heh heh. Hope this helps.