It was then that I decided that I was not going to have shortness of breath ever again and that I wasn't scared of my symptoms, and if I got through a whole day without having shortness of breath just because I decided I didn't have it anymore, than it really was caused by anxiety.
Oh, if only anxiety worked that way.
There's a huge mind-body connection when it comes to anxiety, but for a second, let's just look at the physical part.
The body becomes symptomatic when stress hormone levels reach a certain point. The very simplistic answer to your original question is that the body will remain symptomatic as long as stress hormone levels remain high. Getting them back down to pre-reactive levels can take a while (sometimes a LONG while), because after-all, your body didn't become symptomatic overnight, so stress levels aren't going to go back down overnight.
What trips a lot of us up, I think, is that often, we don't "feel stressed," yet our bodies remain symptomatic, which makes us believe that something other than anxiety is causing it.
This is where the mind element comes in. While stress levels remain high (which can happen for quite sometime), our bodies can and will exhibit stress symptoms, and those symptoms manifest themselves however they want—we don't have control over what happens.
But, we do have control over how we react—which is key. Each time we experience a stress symptom—a palpitation, shortness of breath, dizziness, tingling, you name it—we CHOOSE, whether we realize we're making a choice, to either react fearfully or not. Believing that your heart palpitation is a sign of impending heart failure is an example of reacting fearfully, and each time we react fearfully, we inadvertently cause our stress hormones to increase, which only causes the body to remain symptomatic longer. Conversely, if you experience a heart palpitation, and accept that it's just a symptom of your anxiety and not caused by something awful, you give your body a chance to calm down—and if you do this enough, stress hormones will go back below reactive levels.
So, for you to decide that you're no longer going to react fearfully to your shortness of breath is GREAT. That's exactly the mindset that's needed. However, it's not realistic to say, "I'm going to give it a day, and if it doesn't happen, it must be anxiety." Because again, while your body's in a reactive state, it will remain symptomatic, and you don't have control over what it does. The only thing you can do, when your shortness of breath returns, is to say, "this is my anxiety; it's just anxiety, it can't hurt me, I will accept it as such and try not to worry." That's about all you can do. With ANY symptom.
This is not an easy change to make, by the way. It's a process that can take months, and progress is not a straight line. As you do practice acceptance, you'll find that your body becomes less reactive over time, but if/when it acts up again, it's important to remember to keep accepting the fact that anxiety is behind the symptoms you feel. It's also important to accept the fact that there will be instances that you do freak out, that you do go back to your old way of thinking—again, it's a process, and so may of us anxious types would do well to give ourselves some grace to not expect perfect, 180-degree changes right away. It's a matter of changing the way you think, which can take a long time.
For me, I had chronic, high-level, symptomatic anxiety for about eight months. Even when I finally accepted it all as anxiety, I still had shortness of breath, dizziness, twitching, and a host of digestive issues that lasted for quite a few months afterward before they all fully subsided. Still, several years after this time, I find that my body reacts negatively to stress MUCH more easily than it ever did before. But this is just how my body is now, and I have to accept that.
Mairi, you have the right idea. But don't expect miracles to happen overnight. As great as it might be to think that making that one decision to no longer fear your symptoms is going to bring about instant change, it won't. Making that decision is the FIRST step in the process. I know we all want the fear to go away, the worry, the scary symptoms—of course, symptom mitigation is very normal—but again, it's not realistic when held up against how anxiety truly affects the body. Keep up what you're doing, and have patience. From experience, I KNOW it's possible to get over anxiety, it just takes time.