EXH taking girlfriend to sons wedding

I am in a dilema at the moment and don't know what to do.

Son is getting engaged in January and ExH will be there with OW. When son and his partner told me they'd invited her I thought I was OK with it.
She will also be going to the Wedding in November.

I am still having a hard time dealing with the break-up of my marriage and don't want meet her at such a special time for our son.

I want to tell my son and his partner that I have decided to attend neither the engagement party or the Wedding if OW is going to be there, but can't for fear of hurting my son or losing my son (and possibly my other two kids) from my life.

This whole situation is making me very very depressed. I still struggle with the fact that I have lost my H to another Woman and don't think he will ever come back no matter how hard I pray for him to come back.