You are NOT alone.
Welcome Goldenlady. Sorry to hear that you are struggling; it can be a REALLY intense experience to open up your relationship. It puts a magnifying glass on any cracks in your own self, as well as in your relationship, and it can be pretty overwhelming emotionally at times.
I too have had "nervous breakdowns" around my past fears and upsets that have come to the surface by being "triggered" by polyamory. It takes a huge personal commitment to sort through this baggage and it can feel almost impossible when you're up against the wall of some pretty scary and uncomfortable emotions. You Are Not Alone!!!!! HUGS!!
I think it's awesome that you already know the importance of taking really good care of yourself - nothing can help you weather any storm in life like eating well, drinking lots of water, getting exercise, seeing friends, and engaging in regular routines and activities that have made you happy long term in your life. You CANNOT change the way that you feel, but you CAN change the way that you ACT and the way that you THINK - this will change the way that you feel over time, I guarantee it.
Although this may feel urgent because it's uncomfortable, it's not. You don't have to do anything right now. Is it important to deal with? Yes. But take the urgency out of the equation - you're under enough pressure to deal with what's happening for you already just because it IS so hard to feel these feelings. Be with your feelings. Unpack them. Question them to deah to try to get to the roots of them (there is rarely one). Journal. Meditate. Go for a long, sweaty run or walk depending of your cardio health to clear your mind. Hot baths. Massage. Take time to be with yourself, and give yourself a break from the processing.
Our girlfriend is much more into my husband than I. This has been pretty challenging for me at times, but I also know that it is "the norm" and do my best to share him and the joy and beauty of our relationship without feeling threatened. I don't always win on that front, but all that does is show me that I a) have more work to do on myself b) need more time with my husband. I am really lucky in that my husband and I have an astounding ability to do tough emotional work - I am unlucky in that our girlfriend is not as willing, or comfortable doing that work, so a lot of it becomes my/my husband's responsibility to sort out.
Give yourself space to feel what you need to feel, and work towards not judging yourself for those feelings. They are what they are - indications of areas inside of yourself that need your love, focus and attention to heal. Feel free to SLOW THINGS DOWN and take space with your husband if you need to - there is no rush for things to move forward at a million miles an hour, and a lot of "comfort" in polyamory has to do with pacing that works for everyone.
Post. Post the crap out of your feelings. There are others who feel the same way who will benefit from reading your experience, and tons of people who have been through at least a portion of what you're experiencing.