I've been working in Human Resources for 5 years, and I've asked the "why did you leave..." question many many times. Here are my thoughts on the matter:
Don't lie. If you do, they will find out and that will the end of the process. Not because you got fired, but because you lied to them about it. When I find out that an applicant has lied to me, I immediately think "If they lied about this, what else are they lying about? If they lied in the interview, how many lies will they tell after I hire them?" So the issue wouldn't be that you got fired from your previous job, the issue would be that you lied to me about it. And that's a big thing to lie about. You mentioned that when you tell them that you got fired, they just stare at you. The reason they do that is because they are waiting for you to explain what happened. They're not judging you, they're just waiting for you to say more.
Just be honest - that's the important thing. I've hired people who have been fired from their previous job. We in HR understand that sometimes things just don't work out; it was a bad personality fit, it was the wrong job for the person's skill set, the person was going through some personal issues, etc. You are correct in saying that you don't want to say anything negative about your former employer. But you can tell the truth without torpedoing yourself. You can say something like "my boss and I had some communication problems that led to misunderstandings that caused some performance issues." Or "the job wasn't a good fit for me" (it didn't utilize my skill set / the company's culture made me feel uncomfortable / I hated the job and that caused me to have performance issues..."
Do not say anything about health issues. Don't say anything about depression or anxiety or medication or anything like that. Why? Because it's none of their business. It's against the law for them to ask you any questions about your health, beyond "are you able to fulfill the essential functions of the job?" If they ask you something like "do you have anxiety problems?" or something like that, just turn it around and say "why would you ask me something like that?" Then end the interview, because you don't want to work for someone who asks illegal questions in the interview. That's a big red flag.
I hope that helps. Keep your chin up and keep telling yourself that you will get another job. It might take longer than you'd like, but it will be worth it when you find a place that's right for you.
Hang in there,dobson