You know it's funny, after reading the book "Til your son gets a wife," and after having a happy get together this week with 2 DSs DiLs and my foo I was looking back at the past horrible three years and trying to figure out advice I'd give to Moms starting this process, there is no one size fits all approach, so I'll tell you what worked for me ( so far ).
First of all your relationship has changed. I know you hate it, I think we all do. All of a sudden this person appears in the middle of your life and all contact with your son is filtered through his relationship with her. You are going to be angry and resentful but IMHO the absolute worst thing you can do is show it. Any demands or anger or fury directed at your son's SO will drive him away from all of you. I think you can take that to the bank.
One of two things are true about your son's SO. She is either the perfect person for your son and the one who will truly make him happy, OR she is a controlling barracuda who will cut him off from everyone he knows and loves and make him miserable. We all pray for the first because we want our sons to be happy, but trust me if she is a monster daughter in law any attempts to get her out of your son's life will backfire and actually will give her exactly what she wants. She will point at your family and sweetly say - "they are crazy, demanding, don't love you ...or the dreaded they want to keep you a child, and a momma's boy." Then you become the common enemy and DS is so preoccupied with the "fight" with his family of origin he fails to notice just how controlling or demanding his SO is in every area of his life. Soon his FOO is cut off and he is simply absorbed into his new normal, and he has nothing to compare it to because he has been separated from his past.
I'm not convinced that wasn't the direction I was headed before I found this site.
So what did I do? I became the Stepford MIL. No pressure, no demands, no expectations. Didn't push for holidays, and praised my son and his SO, eventually wife every chance I got, focusing on everything positive I could. Most of that centering on how proud I was of the MAN my son was. Now all of that was true but I SAiD it.
I also took the chance to focus on me, and made sure our sons knew how much fun I was having doing it.
For the time being it's working. Sons are calling and pushing for us to have a part in their lives, still not actual holidays, sigh.....
One thing I should stress though, the original pull away has nothing to do with your son's SO they all do it and it's vital not to take it personally and vital not to blame your son's SO because it would happen no matter who he is involved with....it's a man thing by now I am convinced of it.
We are no longer our son's priority and okay it still bugs me, but I know we shouldn't be. But we are establishing a new and important place in their lives....it's a day to day process though and no it's not easy.