It's not all your fault. It's unfortunately a combination of you both. He should have talked to you that something was lacking for him, and didn't. He took the cowards route, hoping that you will be so mad that you leave him be and "go away" so that he can enjoy a new life.
I'm sorry you feel alone and lost - and that's totally normal, as what you know has just completely changed. Create a new norm for yourself. There's a country song that says "you ain't worth the whiskey". Don't cry over him. Mourn the good times and then start moving forward to create new good times since he is already doing so. The best revenge is to be happy - without them.

So many of us have been exactly where you are. And here we are telling you it WILL get better, even if it doesn't feel like it right now. Get into therapy to sort out your feelings. It will help you a lot. Do for you now. :)

I am in a similar situation and therapy along with the book "Rebuilding when your Relationship Ends" and some meds too for depression and anxiety have gotten me thru the first two months. Good luck to you.

Horrible. Seeing someone behind your back and blames you for it? Says everything about him. Good people aren't deceitful. Manipulative people seek to control your feelings and don't give you all the information so you have a fair chance to understand the truth. How a person behaves when no one is looking is the truth of that person. And thank goodness, "truth will out", even after decades. Sometimes it helps to focus on how a person behaves (when no one is looking) and not on how they make you feel to rebuild your competence. Then ask yourself do you respect such a person?
Try and reconnect to your life/ identity in spaces where this person isn't present, even if you have to go back to childhood to practice a sense of strength. A place where your reality isn't being manipulated/controlled and rebuild experience of trust. A place where friends don't betray you.
Permit yourself to stand up for yourself you won't be alone, reclaim you.

I’m so sorry to hear that. Your post brought tears to my eyes, and wish I could give you a hug. Words seem inadequate to convey my care and concern over the pain which you shared. I just said a prayer for you and I hope that God will provide the comfort and help you need at this time. Do you think it would help to speak with a counselor? A good counselor might be able to give you some solid guidance. Sending hugs & prayers your way!

Same thing happened to me. Completely blindsided. It was hell - still is sometimes. But 2 years later it is easier. You will find a way they this.