Parents sometimes don't fully realize what happens when one of their children gets married and that's one reason I wanted to reprint your letter. Perhaps your words will help other parents avoid your mother-in-law's mistakes.
When children marry, a major transition takes place: They are no longer under their parents' authority. The Bible says, "For this reason a man will leave his father and mother and be united to his wife" (Genesis 2:24). They now have formed a separate family, and parents need to avoid trying to control them. That doesn't mean we no longer love our children or won't try to help them — but we need to be wise in how we do it. Almost every parent faces this problem; someday you will, too.
Is your wife aware of your feelings, and does she see this problem the way you do? Don't let your resentment or anger build up and don't take it out on your wife. Instead, decide together how you can best handle this. She may need to confront her mother and tell her candidly that while you both value her wisdom, it's best for her to keep it to herself unless asked.
The most important thing I can urge you to do, however, is to put Christ at the center of your marriage. Have you made your commitment to Him, and are you seeking to put Him first in all your decisions? Pray for God's wisdom and guidance as you deal with this, and with every other problem you face.