First of all iinkless,
I hope that all of that anxiety of joining our gorgeous community has now passed and that you will be contributing to the posts soon!! Enjoy yourself!!
Now regarding your foundation and the remarks of some of the MUA, I am sorry to read how unkind and unhelpful they have been perhaps you could go to another counter or store. I am assuming that you are in search of something with medium to full coverage since you mentioned circles under your eyes. Now Face and Body is a great foundation, however it gives very light coverage, its really for those who don't have anything to cover up but who want a fresh dewy look, so having read what you've said I wouldn't recommend it to you. What is your skin type? Oily, Dry, Combination??
If you have Dry skin you could try - Studio Tech
Oily - Studio Fix Plus or Match Master
Combination - Studio Fix Fluid (which you've already tried)
Finding the right foundation in most cases requires a fair amount of trial and error, shop around, don't limit yourself to just MAC. Hope that helps.