Hi snapper,
I must agree with the others here... stress definitely has an impact. Be careful, though... stress is not the CAUSE of BFNs and miscarriages and such. But given that it affects your health in general, in a very diffuse way (such as weakening your immune system), it can affect your chances.
It's very normal to worry from time to time. We're not given as many chances as fertile people when it comes to having children. So each attempt matters far more to us than it matters to some people who don't have to go through our treatments. Most clinics offer psychological services for their patients. Have you considered having a session? I'm a psychologist, so of course I'll recommend that!

I think that person would be able to give you tips on how to deal with your anxiety. Talking to a neutral person can also help figure things out, too.
I work with cancer patients. They, too, can suffer from anxiety. Here is one tip I give them. Instead of fretting about different scenarios and "what if", try to pinpoint your worries in complete sentences (not scenarios), and write them down. For exemple, you could write "My treatment will not work." One sentence.
Then take the time to write down all the FACTS that prove that statement. Given that anxiety-based statements are usually based on assumptions rather than facts, it's usually hard to find supporting facts. But do try. Then, list all the facts that contradict your statement. Do ask your DH, friends or relatives if you can for that step. The more facts, the better.
Then take the time to reread both lists. Write a few statements that lie inbetween those extremes. Then reread those middleground statements as many times as needed, when you feel stressed. Here's an example:
Statement: My treatment will not work.
Facts that support: I have fertility problems. Treatment protocols are not 100% successful. My last attempt did not work. My last ER didn't retrieve many eggs.
Facts that contradict: Each attempt is a new one. My clinic's success rate is XX%. I cannot know for sure that the attempt will not work. My doctor told me we do have chances to succeed this time. My doctor adjusted my meds so things go better this time.
Middle ground: My fertility issues make it difficult to have children. Treatements are not perfect, but they do have enough success rates for me to give them a try. Although it didn't work in the past, my doctor made adjustments to increase our chances. I can't know for sure that it will work, and I can't know for sure that it won't either. I did everything I could, my body is doing the best it can right now, and it's out of my hands from now on.
See what I mean? it's better to write things down because it will help you structure your thoughts better instead of going crazy on many scenarios at a time. Writing things down also help you control your emotions and concentrate. Eventually, writing things down also help you integrate the "middle ground" statements, "buy" them, and that will lower your anxiety in the long run.
Whether you worry or not, the result you get is the result you get... worrying won't change that. What it can do is make your life miserable during your treatments...
Hope that helps. Hang in there sweetie, and I'll keep everything crossed for you.
Sophie