It sounds to me, in spite of your anxieties and troubles, you are valued at this old job. This is leverage for you in a way. First think about a new job and the old familiar (though uncomfortable) job. I'm gonna venture a guess that it's possible your anxieties are not related directly to the job itself but working with people etc. or something else.. anyway what i mean is general issues that come with most jobs. Again, I'm guessing but.. with the new job you don't know for sure that the same anxieties might arise and cause the same problems. I know, for myself, my issues and challenges to with me everywhere and i have to deal with them personally and no change of job will probably make them go away. So, with the new job you're taking a risk, you have no rapport with the new employers and are not exactly valued as much -- meaning it may be more difficult with new people and environment. With your old job, you have this leverage where, you at least know they like you and value your work. There is a better chance even if only temporarily, to get over the issues there because of this. Lastly because they actually did come to you to come back, you know that at the very least you might be able to work out something with them if they understand your situation, your issues and challenges. They know you're a good worker and worth investing in, maybe they'll find a way to work out some plan that migh alleviate your anxieties.

Just some thoughts.