You want to know why you're hurting?
(This is my humble opinion).
Because deep down, even though you rationalized it was the right thing not being with her because of her cheating (and moving on without her), there was a part of you that was hoping this relationship with OM would fall apart and she'd come back to you begging for forgiveness. That she had come to her senses, that she was wrong, and all she ever really wanted was you, and could you please take her back.
Yes you mention now that some posters here said the relationship would "crash and burn", and even though you went on without her ("dating and making new friends") deep down in your heart you were just going through the motions as you were HOPING that it would "crash and burn" and she'd come running back to you.
Well guess what that hasn't happened and in fact they are still together and it appears they're taking their relationship to the next stage (getting engaged).
Listen, I/we know how painful this is.....we truly do!!
I know you don't want to hear this, but maybe, just maybe, this is what your heart NEEDS TO HEAR (that she's engaged and moving on).
Your head has already come to grips with all of this. Your mind knows she's a "liar and a cheater" and a "selfish child", but your heart hasn't gotten there. It's still in denial and has been hoping that this has all just been one bad dream and that you're going to wake up one day with her running back to you and back in your arms again.
Again we all understand these feelings.
Maybe now, your heart will realize what your head/mind has already figured out, and that she's gone but more importantly it's been a HUGE BLESSING that this all happened because she wasn't the one for you after discovering who and what she really is.
Your value and self worth but more importantly your PEACE and HAPPINESS has NEVER been attached to her.
As painful as it is, you've hopefully now reached the ACCEPTANCE stage of grieving a loss of a loved one.
It's the HARDEST stage in my opinion to get to but it can be very liberating.
There's a reason the front windshield in a car is bigger than the rear view mirror. What's in front of you is WAY MORE IMPORTANT THAN WHAT'S BEHIND YOU!!!
You have your whole life in front of you.
It's time to let the past be just that.....the past!
Instead of being sad about all of this you should be EXTREMELY HAPPY that this has happened as the days have been wasting away hoping and praying for something that was NEVER MEANT FOR YOUR JOURNEY!!!
Now go live your best life and find happiness and self worth within and not EVER depending on another person or thing or situation to give this to you.
Today is the beginning of your new journey......HOW EXCITING MY FRIEND!!!