I'm sure you were a very good wife. His being an addict is no reflection on the quality of your love for him.
Your mother-in-law is enabling him. She wants to believe the best, even though she isn't really seeing it. She's in denial, you aren't. Don't let that make you feel guilty. You know what's really going on here. You have made a decision based on what's best for you and your children. You have nothing to feel guilty about.
This is a big change for you and your kids. Try to take things one small step at a time.
Hugs,
Gabe