Just because you love someone doesn't mean you have to stay with him. You know in your heart that you aren't thriving here. This is why things are moving in this direction.
The reason you're shaking and freaking out is because you're probably undergoing some form of PTSD. A breakup for any of us in a relationship after the one where we were betrayed by someone else is probably going to bring back feelings that we experienced (even on a subconscious level) that we underwent when we got the initial bomb or went through our divorces/separations.
What you should tell yourself, because it's true, is that the feelings you experience are not just emotional but physical. My doctor has told me about "pain memories" stored in the body...that this is part of a PTSD sort of set of symptoms. They can be overwhelming, and they can probably scare us to the point where we think, I "can't" go through this withdrawal again, I "can't" go through this pain again, because I know how long it lasted before...but you CAN and you WILL. You are first off in a stronger place from before. You bounce back more quickly. And you already proved that you got through the worst of it with all that went on in your past.
The other thing to consider strongly is again the biology of this stuff...you are in a similar situation, this time you are the "breaker-upper", for good reason, and your body re-experiences the same physical sensations...the shakiness, the nervousness, anxiety, panic. But it's biological in so many ways. Think of it as having the flu or something. It's a set of symptoms triggered by the PTSD type memories...and as a set of symptoms, they will go away. In fact, the way that my doctor said you can combat PTSD is to relive on some level the experience that first traumatized you. By reliving the experience or one like it, and doing so with some tools to handle it and therefore handling it BETTER this time around, you are actively taking the steps to fight the PTSD in you. You are really "re-framing" your trauma.
Give yourself a mantra, something simple, and just breathe, count your in and out breath, 1-2-3..."I will be fine", or "Everything will be ok." Something like that. Or "I am stronger than how I feel at this moment."
This is a positive step for you even though it feels like hell. You're moving in the direction that shows the ultimate in caring for yourself first, and it's scary as anything to make that move, but pull strength from everyone here who has done it and moved on to a better place.
M45 Bomb 6/09; EA 6/10; Divorced 1/11 Proud single mom of 7 little feline girls and one little feline boy "Fall down 53 times. Get up 54." -- Zen saying
