Its okay to feel that way, I still do. We all do with our S.
The part you shouldn't do is "Wait" to see what happens w OW. I already gave my blessing to my W and hope it works out w OM. Since then, i haven't worried or waited for that. Heck if she even brings up dating, I simply say "Great, I am happy for you", then i bring up a new topic or walk away.
Remember, your S fell in love with you before because you were a special person to him, you had good qualities about yourself. You just need to focus on yourself to bring those qualities out for yourself. Everything else will fall in place.
So top worrying or waiting for the OW. Heck if you don't want to openly tell him have fun, at least in your head, tell yourself "Who cares"
Your only in the Sad phase, as long as you let it continue . My sitch is only 2 months but i realized, there is no reason why i have to be sad. I was originally sad for my kids too...but i realized my W is doing this, i can't stop her, so i can't be sad for the kids, i need to be the best dad in the world.
I am already mentally preparing for the day, my W moves out. Heck, I am planning weekend events for myself already. I am planning Dinner with my kids, fun stuff, movie night. Things i will do without my W. I already thought about taking my kids on vacation without my W. In the end, she can join me on the fun or she can go get her own fun.