A similar situation happened to me about a month ago, and I feel your devastation as I'm still wading through my own. So many similarities to your story- I feel so empathetic for you.
I too was betrayed by a new love. This man was so wonderful, seemingly perfect for me. We bonded very deeply. Turns out he's had another girlfriend the entire time he'd been telling me he loved me, while we'd been planning our future together. Out of the blue, he came to me and confessed that he was breaking things off with her to be monogamous with me. I was dumbfounded and shocked understandably as right before we'd gotten together he said he'd decided to take a break from dating anyone else. But he apparently lied about that. He claimed they were just friends with benefits and he wasn't serious about her like he was with me. They'd been on and off for a year.
After a week of hardly eating, weeping every 20 min, shaking that wouldn't let up, I decided to forgive him. He was so sorry, he seemed so reticent and sad and reminded me that he chose me. And I tried to get through it, the fear and betrayal and anger and appalling sadness. He was there and attentive during this process, but I've lost so much weight, get horribly dizzy everyday, and even my job has been affected because I can hardly concentrate.
I told him initially that I didn't want to ban his friendship with her, but that I didn't want them meeting alone, going out together, or to meet in person without him talking to me about it first. This past weekend, he invited her to his house (where they were alone together), casually texted me that they were together and 'having a good chat', and then when I called him out on his promise to loop me in, he got defensive, angry, and refused to speak to me for a day. He said that this girl deserved an in person apology (she thought they were pretty serious too), salvaging the friendship meant a lot to him, and he was tired of how supervised he felt, that he'd do what he felt best.
When I got that text that they were together, I was just headed in to my work holiday party. It was so embarrassing to refuse to order anything, just sit miserably, trying to drink water and shaking like a leaf.
You were so much smarter than me, OP! Maybe I shouldn't have given him another chance. Should have cut him out of my life. This second betrayal hurts worse than the first. I totally understand how awful you feel now, how overwhelming the physical symptoms are, but you will get through them given some time.
Other people have mentioned that you are grieving the 'death' of the man who would never betray you, the one who you thought existed in him. I resonate south with that. It is such a physical shock to have reality shift so dramatically.
I don't know what to tell you about the physical stuff, as I'm struggling with it now myself. What has worked for me. Schedule time in bed where you can relax, be cozy, and distract yourself with something, movies, knitting, writing a journal.
When no food is appealing, or is vomit inducing (thrown up more times in last month than in last 5 years), bulletproof herbal tea can help. Blend coconut oil into hot tea, or just use heavy cream liberally. Go for dense caloric heavy things, clear your mind, and eat a few bites. Remind yourself you deserve to be fed, you need to nourish your body so you can get through this. Focus on counting breaths when you get hysterical. It sounds useless, but can help. I like the bath idea mentioned here, should do that myself.
Feel free to PM me. You said you felt isolated right now, and since I do too and we are going through something similar, keep it in mind. Hang in there. I hold onto the hope it will get better