Maybe the problem is not that you are an overall bad worker but you are still in the wrong job. Perhaps all these different jobs haven't really been that different because they require the same transferable skills, such as being detail oriented or highly organized. Or maybe these jobs have asked you to multitask when you may actually perform better at a job that would let you do one thing at a time. Perhaps if you can write down throughout the day under what circumstances you do your best work then maybe you can communicate with your current supervisor a bit and talk about creating that kind of environment you need so you can quell those current worries. Even more-so, it may be a reasonable to take some time to find out what your skills really are. Maybe your work output should come from what your hands create, or come from an ability to deal with people....

I really liked this book by Nicholas Lore called "the Pathfinder". I found it in Barnes and Nobel. Maybe flip through it and see if it can give you any ideas.

I relate, by the way. I have this problem at work too, I'm trying very hard to move into a new field all together!