Hi, I'd like to help you with your question.
If you are feeling controlled now before marriage, then you have a reason to feel concerned.
You are right, people do not change after marriage unless they really choose to. If he says he is willing to change after marriage, why not change now? It sounds like he wants to marry you so you will have a harder time leaving the relationship if you need to.
One of the best ways to help yourself is to learn what you can about emotional abuse, which is what controlling behavior is. Here are some resources to help:
The Manipulative Man: Identify His Behavior, Counter the Abuse, Regain Control by Dorothy McCoy
The Emotionally Abusive Relationship: How to Stop Being Abused and How to Stop Abusing by Beverly Engel
Why Does He Do That?: Inside the Minds of Angry and Controlling Men by Lundy Bancroft
You can find these books on Amazon.com or your local library may have them for you.
Emotional abuse/controlling behavior is one of the most damaging behaviors in a relationship. It does not leave outside injuries or scars so it is easy to dismiss. But the damage it does is extensive.
An emotionally abused partner learns that she is not important. She cannot have wants, desires or any needs met that aren't controlled by her partner. She learns to minimize herself and think of herself as not important. The abuser will continue to escalate the abuse in an effort to keep his partner under his control. If she should rebel and try to stand up for herself, he punishes her. The control may eventually become so severe that friends, family, jobs and even dressing herself or picking her own hair style are controlled. The abused partner becomes totally under the abusers control, not able to make personal choices of her own. The abuser becomes so fearful of losing control that he takes over his partners life completely.
Once in this type of situation, it is hard to get out. Help is needed through women's shelters, friends, family and counselors. The damage to self esteem and personal space can sometimes take years to repair. The man can also become violent so law enforcement may need to be involved.
Although this is a worse case scenario, it happens more often that not. And predicting how your fiance will act once you are married is impossible. So educating yourself now and deciding based on that information how you want to proceed is the best option possible.
I hope this has helped you,