I am 16 years on from the awful time when my ex fucked off with the other woman. He didnt wait any time at all before introducing the new set up because she was my "friend" and her boy was my son's friend, so they already knew each other and he moved in with her within days of me telling him to go when I discovered their plan to find somewhere together.
She was pregnant almost immediately and yes, it felt awful. I too felt as if my life with my son and my dreams of family life had been stolen and handed to her on a plate. When exp described his new family as "blended" I retorted with "you mean liquidised- smashed to bits and left as an ugly mess". Over the years I have moved from feeling the loss so much, I began to quite like the regular friday nights free and every other saturday night too.
As my son got older I began to get insights into their life via my son- she is a crap cook and my boy hates eating there, he also thinks she is a crap parent- far too permissive and needy. Recently they got a dog- my exp hates dogs, now I know that he got what he deserves- a house overrun with pet mess, kids and crap food. The feelings have moved from loss to indifference about him. I would still only piss on her if she was on fire if I could be sure to add insult to injury though.
Keep on keeping on OP, you will get stronger and your children will grow up. If the OW has babies they will be stuck in with the babies whilst you can spread your wings and fly. Forget karma though- it does not work like people like to think- you will wait until hell freezes over to gloat over their/his/her misfortune, so get on with your life in the bits of time you have that are yours- ALL YOURS.
Bite back any tendency to ask questions about the holidays they have and just mutter "that's nice dear" if the children are desparate to tell you about them. Find another single parent to do holidays with. We managed to have great fun sharing the load with another friend and her son over many holidays camping in the UK and futher afield.
Dont beat yourself up about being bitter- it is only those who have never experienced the utter devastation of this kind of loss who use those kinds of expressions. You are not bitter- you are hurting and it is real, and it will get better. Really, it will.