My views are that she is the only woman in my life and therefore i can only play all my fantasies with her.
So, what happens when your fantasies begin to include another woman? A man? Or maybe bleeding? Or asphixiation?
You say that she enjoys it as long as you don't go too far, but then you say that she tells you that you are going too far. Do you know what this sounds like to a fair minded person? It sounds like it is entirely possible that you are abusing her and she is only enduring this to keep the peace in the relationship.
When you are intending to cause physical pain to another person, and that person is physically weaker than you, it is important to enter this in a way that causes zero pressure to the other person. Believe it or not, it is narcissistic to think that you are entitled to live out every fantasy that your mind can conjure. If she does not enjoy it, then it becomes abuse. Once you put her under the pressure of physical abuse, then it is nearly impossible to find out where her threshold truly is because of her own fear.
There is one aspect where you are making yourself vulnerable that you apparently haven't considered. At any time, she can pick up the phone and legally call you an abuser. When you try to tell a lawyer that you have a right to live out your fantasies, they will remind you that you are living in the real world.
I'd suggest that you spend some time focused completely on her fantasies. Make her know that you love her in a giving sort of way. Then, if she begins to ask for a little pain, you can let her tell you how much is enough. In marriages that aren't dominated by one person's fantasies, this is called mutual respect.