I helped my parents to not continue parenting me. When I got married, I told them not to interfere in my life. It took a couple of times to get the hint across, but it worked. I wanted their friendship, but their advice was really not very welcome with me. After about two years, we settled into an excellent relationship that has continued for the past 33 years.
Now, my children are grown and married. I have told them both that when they need me, I am here. They know that if I am invited that my help and advice is always available. It is hard to put a button on my lip at times when I see their husbands not acting as I think they should. However, I am convinced that most marriages fail because outside influences and especially parents intrude too deeply into the relationship.
Your adult children will make mistakes. Learn to accept this. You got to make your mistakes. Now, let them make theirs. It is tough. You can do it.
The first step to letting your adult children live their own lives is to just stay away from them. This is not avoidance. The reality is that you do not need to call them everyday or visit five or more times per week.
Wait on them to call you. They will call when they need you. A phone call that does not include interrogation once or twice a week is good. Inviting them over for a meal three or four times per month is great. Do not bake a cake just to go see if the house has been cleaned. Resist driving by to see if the grass has been mowed.
When you are at their house, try not to inspect cabinets and closets. They have doors for a reason. The doors keep you and other nosy people out. It is really none of your business if the bed is made or the floor is swept.
I know this is difficult for some to grasp. Becoming a pest to your adult children will only alienate them. They will come to dread your visits and phone calls. Do not try to pump the grandchildren for information to be used against your children.
Adult children no longer need the control or close oversight of a parent. They need an older friend they can trust when they need help or advice. Your children want their parents. They just do not want them running their affairs.