I know how you feel. It does get better, I promise, but it will take time.
I agree do something nice with your DC, they are more important now, and remember that eventhough DC will have contact with OW, they will never have the same relationship with her as they do with you.
My ex left 18 months ago, moved in with OW straight away, he'd been seeing her 3 years, I hadn't a clue.
They have lived all this time in her small flat, while I'm in the 5 bed family home. However, in 2 weeks time they are moving into their first home they will own together.
I feel sad about that because it's another step for them, they are getting their life sorted. I'm 
I have a nice home for now, but it's up for sale to fund the divorce settlement. I will not live in such a lovely home in the future but it will be mine, no one can take that away from me again.
I have a new lovely man, I see him a few nights a week, my DC like him and we have a great time together. So it's not all bad.
But, yes, it's tough. Keep smiling.