Thank you so much for your response! I didn't realize that the Nimitta subject was something controversial.
I just began doing some research on what I was experiencing and this idea of Nimittas kept coming up. I have seen the maps that you suggested earlier this year and also own Daniel's book (2nd edition). I also have read the short book by Mahasi Sayadaw, titled Practical Insight Meditation.
I learned about Nimittas while researching my symptoms online and also through a book that I recently purchased called Mindfulness Bliss and Beyond by Ajahn Brahm. The way that he explains the experiences leading up to the
lights that I speak of are pretty spot on.
Following the breath, a feedback/building of concentration, a shift in perception, effortless focus on the breath accompanied by an equanimous bliss from the relief of some letting go of the senses followed by this overwhelming light that completely takes over my experience. Of course then I get the palpitations. I will give your suggestions a try, by the way!
I have been trying to use the insight maps that Daniel created, but I feel that they can a bit broad and ambiguous. Even looking around this forum, just about anything that anyone experiences is labeled as A&P. I don't mean this to be critical, only that I feel that it may be too broad. I find it difficult to come to terms with the idea that after 100s of hours on and off retreat, practicing very diligently, that the entire broad range of my experiences have all simply been Arising and Passing Away experiences.
With complete honesty I really feel like I have been all over the map, past the stages of A&P. But I do think that I've also regressed to it quite a few times. According to the way the A&P is described, I don't think that I have ever been anywhere before it. At least not since I was a very small child. This is not meant to be a critism of your diagnosis! I am really airing grievances more than anything else!

That being said, I know that I have a ways to go before achieving any level of permanent reduction in suffering. When I got off retreat earlier this year, I felt so incredibly peaceful for about a week. I remember that nothing could touch me. It felt as if 'this is the new me'. Then it all went away. In some ways I wish that I had never felt this way because it has caused me to suffer even more now that I don't have it. In other ways, it has made it easier to imagine what a particular level of peace can feel like.
Essentially I am at a point where if I can't figure out how to break out of this cycle, I may just take a break from meditation for a while. This plateau of cycling between the most beautiful thing that I've ever seen to fear, to excitement, back to mundaneness is just exhausting!