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 me I 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.