Okay well for one- NEVER cry at an interview and NEVER NEVER EVER badmouth an old employer. Those two will guarantee you'll NEVER get the job.
Seriously girl I've interviewed and hired people before and I would have thrown your resume in the trash if you cried at your interview! I'm not trying to be harsh, but really it happens, people get fired, it doesn't make you a bad person or unemployable!
Believe it or not- the TRUTH is the best answer. What you said is fine. "A customer needed a fax, I sent it to her as soon as I could, but she called and complained that I never sent it, and I was fired."
If they ask anything else just remember to not bad mouth your employee, and if they ask anything like omg really, why would they fire you over a fax you can just say "I honestly don't know, but that's what happened."
One other suggestion is google "interview questions" and go through as many as you can. With this job and my last job I aced the interview this way. I was prepared for 99% of the questions (they all ask the same thing) and I practiced with my sister till I was satisfied with my answers, and then any questions I hadn't heard I was easily able to think of something to say
One other suggestion is if you are nervous, just try to take a deep breathe before the interview and calm down. Also don't ramble! Just answer the question. AND when they say "tell me about yourself" don't tell them random things they don't care. Tell them about your character, you are a hard worker, something. I swear I had people tell me things like I was interviewing them for a relationship. I was like I don't care what you do in your spare time! lol.