One of the hardest things is to just let them go. They are married adults now, not our little kids any more, and as much as it may hurt we have to let them make their own mistakes. So, my #1 rule was always "Don't interfere!" Not with words or attitude.
#2 is "You're not the important one in his/her life now, the spouse is." And that's the way it is supposed to be.
It's OK to worry if you keep it to yourself, and never stop loving them!
What great advice Claybird. I wish my mother-in-law had read that when I got married - especially rule #2. I also think that you need to love, and accept the person that your son or daughter has chosen as a spouse. You may not agree with your son/daughter's choice of spouse, but if they are happy that's all that matters.