I really feel for you, @Pleasenotme . It's a special kind of grief - he's not the wonderful soul mate you thought he was. And because he's still alive and he's still being nice to other people, they're not grieving, so you don't have that solidarity in missing him. And because he's not actually dead, you don't have the certainty he's not coming back - in theory, he could.
In reality though, even if he came back, he wouldn't be the man you thought he was. So, you have certainty there. The man you thought he was wouldn't have tossed you aside for an OW. So, even though there is someone walking around with his face, the man who you love is gone.
Your extreme reactions are normal. You were together for such a long time and for that part of your life to be over is a huge adjustment. Don't let anyone tell you that you're overreacting or you should behave a certain way. Your feelings are valid.
I do think speaking to a counsellor will help. The people around you won't be able to relate to your feelings of loss as, to them, he's still there. A counsellor, who is more removed from the situation, won't have that confusion. And I promise, they are trained to be non-judgemental and they have heard all sorts.
And you know what, if anyone was to be judgey - you're not the villain in this story. You know that. You have nothing to be ashamed of, and you deserve as much support as you can get. Get those free counselling sessions through your workplace EAP. You're entitled to them, and you're entitled to start feeling better.
I am so sorry for all you're going through and all of the horrible twisty feelings. I wish I could do something to help. But the only real answers are time, letting out the feelings when you feel them, and talking to a professional. Therapy is not a sign of weakness, promise - accepting that help is a sign of strength.