I battle the same thing - fortunately I was able to use short term disability to get my feet back under me. I know that is not an option for you I will be heading back in less than a week and have the same concerns. My job is a high pressure, high stress, "I needed this yesterday" kinda job so I can relate on some level. Breathing helps me if I catch the stress/emotions early enough on. Also, sometimes I just close my eyes and say I can do this to myself (sounds dorky, but it helps) The last thing I try to do is not think about it as an 8 hour shift or a 10 hour shift - I tell myself, just get through the next 30 minutes and you'll be fine...over and over. Sometimes the above work, sometimes I'm in the restroom crying, and sometimes I just have to leave early (yeah, my attendance is terrible). If I can find something small to buy myself some time I can usually keep it under control.
Sorry you are going through this, it is hard to hold down a job when you feel like everything is falling apart. Hang in there!