Having to leave a job full of stress

I have gone this round before with jobs in my profession. It is crowded with people and the schools keep pouring out more people. Now, they hire you and try you out, and if you do not dazzle them, you are out in 2 weeks.
The stress has become more than I can bear. I accepted a job that is part time, but I was told I would be trained, but the boss went on vacation for 2 weeks, and I was at the mercy of the coworkers, who were to train me. Most of them were a nice enough bunch, but I am not where they are professionally, and my inability to work at their level has made me lose my confidence day after day.
The stress is affecting me, I already have an anxiety disorder and am on meds for it, but I am realizing this is affecting my heart (my doc ordered an echo because of a bad EKG) and now, I am on Metformin.
I feel like I am killing myself. I typed up a letter to HR and my boss saying this wasn’t a good fit and thanks for the opportunity, and you can have the 2 weeks notice if you want me there. (They really don’t need someone else there, so I bet they tell me to go.) I have wrestled with this for days, and I decided I cannot go into a place and feel that level of stress.
I have talked about this with a friend of mine who has a lot of experience in this. I told her I do not want to be the Sarah Palin of my industry. She understood and said a lot of what I was expected to know, she doesn’t know.
There are not many jobs out there. One employment agent told me she is barely making ends meet finding people jobs now.
I am still looking, but looking outside my profession. I put a lot into it. College, licenses, studying, continuing education, but it seems you get these tthings and some experience, and they want more and more out of you. They want someone with 2 years experience to know what someone with 10 knows.
This is a very difficult decision and I have to find another job. I have spent nights applying online to places and make some calls during the day.
I am feeling I should go in Monday and turn in the resignation letter. I am not getting better and my anxiety is causing me to doubt myself so much. I am sure this is not helping my diabetes either.

Had to vent.

Thanks, this is very difficult. I know I am not what they want. It is impossible for me to be that advanced with my amount of experience, and this is wearing down my self-confidence.

The job market out there is horrid, so it is like stepping out to nothing.

I told myself, this isn’t what I want my life to be like. Now, realistically, it is very hard to have just what we want, but this stress is eating me up.

Thanks for your words.