How to code on a very tight schedule?

Question

I'm working on a project that has very tight schedule. I don't have much time to code and test (even though I work more than 12 hours everyday, it's still delayed), and the result is very fragile. Its code is also very dilemma.

This program is used by all offices in our customer's company, which is located in many countries. I regularly get phone calls at midnight about errors from our user/tester or about them not knowing how to use some features.

After three years on this project, I feel very stressed and I can't sleep well because I'm very worried about errors and phone calls.

I have a few questions:

1. For three years, all the code I've written is just the perfect usage scenario code (so it break easily). It's poorly designed and doesn't have any unit tests. I have lots of problems because of this fact. Therefore, I want to know whether is it's feasible to write code that works when the project has a very tight schedule?
2. How can I write better code in the same amount of time?
3. How can I clear my mind and don't get worried about work when I go to sleep?