I’ve been there and felt just like you do 
and survived.
The physical pain and breathlessness will stop soon, I promise. For me, the worst of it lasted about a week. It was like having a panic attack 24/7. It felt like there was an elephant on my chest.
Six months from now you will look back on this and all the emotion will be replaced with clarity and perspective.
For now, go back to basics and focus on getting through each 10 minute period at a time. Concentrate on your breathing. Put on a meditation podcast. 20 minute meditation exercises got me through the worst, even if I had to play them back to back. Look up Tara Brach. Her voice is very soothing and her podcasts helped me tremendously.
If you can’t eat, make sure you’re drinking enough. Try a smoothie or a cup of hot chocolate. Grab a bag of crisps and eat just one. See how you feel, then try eating another.
Be your own carer. Imagine yourself as a child in distress, not an adult. Soothe that child, give her the love and care she deserves.
This is just a wave in a vast ocean. Go with it. Don’t try to fight the feelings but accept them. Cry if you need to. You’re allowed to feel pain, and how you’re feeling is understandable. You’re in shock.
As for your ex, he hasn’t moved on. He just isn’t able to be alone because he’s empty and needs someone to fill the void. He’s empty so needs to dominate people. I know your imagination must be running wild, but in truth the reality is likely to be extremely dull and he will almost certainly go on to abuse this other woman, repeating the same patterns. You, however, had the courage to leave and you should be incredibly proud of yourself.
Ask yourself why this has hurt you more than the end of your 8+ year relationship. I think there’s something in that.
Hang in there. I thought I would die from the pain but here I am, happier than I ever was when I was with my own abusive ex. I’d give you a hug if I could. 