It looks like you need a second opinion and I understand your concerns. First, I would like to say that I agree with the previous posters comments. I want to reply because it seems like you are in need of verification but also because I know what it is like to have an anxiety disorder and some skills I have used to help me keep it in check.
First, as stated before, you're not going through withdrawls - at least not physically. The reason your heart rate and shortness of breath occur at night is because you are hypersensitive to your internal "anxieties". During the day it is easy to distract yourself because of all the activity that happens, but at night, there is not so much stimuli so you "feel" panicy more.
What happens is that your "think" yourself into a cycle. The cycle begins as soon as you wake up in the morning because at times during the day you worry about your "panic" at night. As it gets closer and closer to bedtime, you probably already have begun to fear going to sleep. Around this time is most likely when you would have taken Xanax (or, depending on the person, used whatever other "means" to dispell the anxiety) and you would have felt fine. I my experience, most of the time, someone falls asleep before the medication has had time to work effectively - it's the thought process that gets to into that state.
Now, I want to stop right here and tell you that you are not going to die. I know this is the reassurance you are seeking, but the truth is you might be satisfied with the answer for a day or two but eventually you will find ways to think of other health issues or other concerns to keep you up. It's just the way the disorder works.
It would be great if there were an easy solution to your problem, but there isn't. It's not something you can "think" your way out of, at least not without support, time, and real effort. So, if you haven't or aren't receiving medical help, then the disorder probably isn't done interfering with your life. Once you make a determined effort, then you'll begin the process of getting better.
So, for the time being, I would like to give you some tips. When you lay down at night, start some breathing exercises. First take a deep breath through your nose and hold it in for 10 seconds (it's important to count, it helps with distraction). After 10 seconds push all of the air out of your lungs through your mouth. I really mean to push. Use all of the muscles you can including abs, diaphram, and lower back muscles. Make it your goal to get as much air out as possible, then hold it for 10 seconds. At this point, you should be able to simlply relax your muscles and new air will simply rush into your lungs. After a few minutes of this exercise you should feel a little better.
Another technique is to cross your arms across your chest so your left arm is at your right side and your right arm is at your left side. Now, alternate counting using your fingers. So, your would count 1 on your left hand (first finger) then 2 on your right hand (first finger) then 3 on your left hand (second finger), 4 on your right (second finger). When you get to 10 keep going - just start over again. If you notice that you begin to count "automatically" without having to think about it, then change the order. You can reverse it, count by 5's or 6's (i.e. 6,12,18,24,30,36, ect). You can count on one hand twice and the other hand twice before switching - the point is to confuse the two hemispheres of your brain - when they are busy trying to figure out which hand is doing what, your brain will "forget" to worry. If you don't know what I'm talking about with this previous sentance, don't worry (no pun intended), it is not important.
I hope this provides you with some level of relief. If you need more tips or simply want to let me know how you're doing, feel free to send me a message.
Thanks for your time.
-Pete