Oh you poor thing. Everything you're experiencing is normal, the physical pain and messaging him too. I feel so upset and angry for you that you're in this pain and you were messed about.
My advice at the moment is to get through each minute, hour, day and just focus on surviving. You won't feel better in the next few hours or even days, don't try to fix yourself. Let everything out, cry, scream, walk for miles, eat loads of ice cream, anything you feel like doing just do it. Even if that means ranting at him in a message you'll never send.
I don't know why he changed, but you didn't do anything wrong and you know that. You simply at this time are no longer compatible and that will be for the best (although it doesn't feel it now). No answers to your questions will change the situation.
Focus on comfort, distraction and survival. Then, one day soon you'll feel a subtle change where you cry less, think about him less, smile and laugh and you'll feel like you're going to be okay. When that happens you can start to re-plan your life. That stage in breakups is usually when I would dye my hair, buy a pet and plan a holiday.
When you're in an anxious spiral, deep breaths (even if they're few and far between), tapping yourself with a hand on the leg or finger on the cheek, repeating a phrase like "I am okay, I am safe" and sip cold water. Grounding techniques could really help you.
You're welcome to message me if you want, but keep posting on here so we can support you x