The problem is that in a very short time, the boundary is tested again as if there was no understanding from before.

yes your W is quite feminine isnt she. \:\) Stop seeing it as a problem. That is your choice. Boundaries being tested is not a problem. Its an opportunity.
The more consistant and stronger your boundary, the less frequent the testing will be. you let it slip and its going to get pushed back as far as it can.

seriously why would she understand it, if you arent serious about the boundary and consequences?
A consequence does not always have to contain actions. People trying to implement boundaries, are always looking at the 'OR WHAT? consequence, before they even try just stating them. Your W does not require that level of 'OR WHAT' action consequence, boundary.

Words said in the moment, with the serious intent shown, and honest feeling, are oftentimes plenty. I dont know why its enough, but I appreciate it. Just because it takes more then words to get thru your thick skull, (mine too) doesnt mean she is the same


That's when I get pissed and I usually withdraw because I'm afraid of my anger.

your trying to suppress yourself and hide the real you (ooohhh scaryyyy *shiver* ) , instead of authentically having your feelings.
so use courage to face the fear of losing control of your anger. feel your anger, use courage to overcome your fear, by remaining where you are and NOT withdrawing.
Show your displeasure. Do not loose your anger upon your W. Do not withdraw. The more you handle your anger the less fear you will have of losing it.

Its your emotions. feel it and control it, and determine what needs to occur to prevent your anger from occuring again. The boundary. Enact the boundary. If the boundary is breached, you are going to feel anger again.

Its there for a reason.