I have the same problem (breathing anxiety).
Step 1:
Don't do any breathing exercises. Most relaxation exercises start with this, saying "focus on your breathing". That obviously doesn't help us!
Step 2:
Try to take your mind off of it. Try EFT "tapping" (google it). Try chamomile tea. And get exercise. DO NOT DRINK ALCOHOL/CAFFEINE. Also try taking your own pulse instead of breathing exercises.
Step 3:
Keep a journal. Record problem times AND good times. Record your bpm in it during both.
Step 4:
Pray.
Good luck.
I know how you feel! I've only been dealing with bad anxiety for 2 months and am sick and tired of it. I just want to get back to the way I felt before this anxiety kicked in. I also went through the feeling like you can't breathe phase. I constantly felt like I could not take a full, deep breath. I felt like I breathed in and stopped short of taking a satisfying full breath. Completely freaked me out and made me fear going to sleep at night. Thankfully, I have gotten past that point and feel much better about my breathing.
One thing that I was asked by a doctor when I explained my symptoms was if I had been more gassy than usual or had been eating food that produces gas (which I really hadn't). She asked this because gas can build up in your intestine and press against your diaphragm, making it feel like you can't take a deep breath in. She then suggested that I try Maalox to reduce any gas. I don't know if it was the Maalox that did it, but after taking it for a while, my breathing started getting much better and relaxed.
Also, have you done any research on beta-blockers? I personally have not tried them, but have heard stories of them helping many people with anxiety (my aunt is an internist and has prescribed beta-blockers to her patients).
Hope some of this helps! Keep me posted on how you're doing.
You sound like you might be moving into the depressed part of the anxiety/depression cycle. Yes some people struggle with anxiety for years, but most of us have episodes and great hunks of time in between where things are peachy. Plus there are some great treatments nowadays, medications or behaviour therapy.
This is all part of growing up in many ways. Life can be hard and you are gaining so much insight and experience for yourself and others that will help you for the rest of your days.
If lexapro was prescribed and didn't suit you have you been given an alternative medication? There are older style anti-depressants if SSRIs are not for you. Please keep in contact with your doctor. You will get over this and have a rich and full life to look forward to. Not 100% happy nor completely worry free, but who can have that?
Take care.