Decision Making: Quit My Job and Refund my remaining one month or not?

Last March 12, 2013, I have accepted and started a guaranteed 8 hours per day full time job as a content manager, spinning articles and do blog posting. The employer gave me three months advanced payment of $1,200 ($400/month). I even bought a brand new computer in order for me to work smoothly with them, using the money my boss gave me.

But it seems on what I have experienced for the past few weeks, it seems that I am not comfortable anymore in my position, as it is full of pressure and affects my overall anxiety, and even my boss complains about the mistakes I have. Well that's normal, but never been experienced doing pressure jobs before and getting complained. Unlike my previous client, I am only working part time with no time restrictions and no stress at all, which makes me comfortable, only thing is that I don't know if my previous client really did guarantee me for long term basis. In a few weeks, I may be almost 2 months working with my current boss.

So this is some kind of a gamble for me now to decide if I opt out on my third month and refund only $400. I can't take it anymore though, I need to be free from stress for a while, but my money might be back to where it started, and work comfortably with my original "stress free" and "no hard complains or punishments" client. Or I might hang on until the third month and opt out without any refunds? I still have more than a month to go.

Guess I was wrong, I could have never accepted this job in the first place, I made a very bad decision. This is one of the I only accepted this because I need money to get back what I have lost last time. But I think this is good enough for me to start all over again, and I have no more second thoughts doing long term with my current boss.