Were you in your last job for a long time? If so, that might have something to do with the way you're feeling.I am in talks with a couple of people at my old job, but I would feel like such a failure going back or that people would be really disappointed in meI just keep getting things wrong and feel like I’m annoying my new manager by messing the simplest things up!
Starting a new job is hard for most people; as well as getting used to doing things a certain way (even if the job itself is much the same as you've been used to), you have to get used to personalities, politics ... ugh; it's hard.
Try to remember the reasons for leaving your last job and what made you accept your new one - make up a list if you can, to refer back to when you're feeling down.
I returned to a place I'd left about three months prior (this was in the '90s). It was a huge mistake and I looked and felt foolish.