You are over-analyzing your body which anxious people do because they worry that clues are out there and need to be paid attention to, however the road to peace involves focusing your mind on nice things that actually happen instead of a bunch of unpleasant "what-ifs" that have no answers. Not necessarily easy (and in some cases not even possible) to do, but that is the path to peace.
The GI doc said you have no problem, so try to accept that instead continuing to micro-analyze every movement and burp in the process.
The cardiac people said there is no problem, so again try to accept what the experts have tested and found - no problem.
There is no defined path for anxiety, and it is best to avoid meds unless your therapy is ineffective, which you are worried that it is. Perhaps you need to change the therapy to focus on accepting the diagnoses above in order to work on the cause of the anxiety, namely fear that your body is plagued with a health problem that the docs can't find.
If you are too anxious to make progress with the therapy then meds might get you in a calmer zone for a while so your therapy can be easier to absorb and accept, but the nasty thing about anxiety is you the victim will have to make that decision (at the inopportune time when anxiety is biting hard), since as I mentioned there is no defined path to dealing with anxiety.
Just to let you know, medication doesn't fully fix these problems for most people either. My own view is that medication is necessary when your life is severely disrupted and therapy hasn't worked. Your description is more of a problem with stress, as when the stressful event passes your fine again. This is a pretty common problem with humans. I went on meds because I was getting serial phobias and my world was getting very small and I had tried therapy with different therapists for a long time. It was my CBT therapist, in fact, who despite not being in favor of meds told me that was probably necessary for me. You have to decide if it's the route for you. As for your digestion problems, that's a pretty common response to stress, and stress if pretty much a self-inflicted problem -- it occurs when we let others define us instead of us defining us. Generally this is a problem for therapy, but sometimes it takes a few shots at the apple before you find one you click with or find the right type of therapy for you. In your case, it might be nice to know where this insecurity comes from that makes you so stressed about performance and CBT therapists generally aren't interested in why we have anxiety, only in techniques for dealing with it. Again, it's up to you to make this decision. Whatever you decide to do, don't give up on therapy, though. Good luck.
CBT never worked well for, I've always had the best luck with medications. With that said, Lexapro isn't a great SSRI in my opinion. I have been struggling with severe anxiety and panic attacks for about a year and half now. I think I now finally on a good combo. I am now on 225 mg Effexor, 20 mg Prozac, 1 mg Rexulti, 200 mg Lamotrigine and 300 mg Gabapentin 3x a day. My anxiety is finally starting to ease. The only problem with my combo is how many serotonergic drugs I am on. Too many serotonergic substances can cause a potentially lethal condition called serotonin syndrome. I tolerate serotonergic drugs well though and have never had an issue with it.
I would suggest to your doctor about maybe trying Effexor. Some studies show Effexor to be superior to Zoloft, Paxil, Prozac, Lexapro and Celexa and similar in efficacy to Elavil and Imipramine (older TCA drugs with more serious side effects). You could try maybe a combo of Effexor and Rexulti. The Rexulti acts as both an alpha and beta blocker as well as a seratonin dopamine modulator. The alpha and beta blockage keeps my heart rate in the 80's instead of 110-115 that it was. It has also eliminated my heart palpitations which has greatly helped with my anxiety (I am cardiophobic).
I am, by no means, saying that CBT doesn't work but a lot of times medication is required to get an adequate response. The unfortunate thing about medication is that it can a while to find the right medication or medication combo that works. Gabapentin, while not technically approved for anxiety, helps significantly. A lot of psych docs prescribe Gabapentin off label for anxiety and panic disorders. The problem with Gabapentin is that it can cause dizziness and sleepiness in some people. I tend to tolerate it well though. I am actually prescribed Gabapentin for chronic pain but it helps augment my current meds. I'm sure it's not the same for everyone but Lexapro made my heart palps worse thus increasing my anxiety.
I also find that Omega 3, Vitamin D3, Multi-Vitamin and a good probiotic make a HUGE difference.