If it was a boundary, they are bound to be...stretched. You didn't push it that far.
If she can't handle being in an open relationship she shouldn't be. Her concept of her going first is proof she isn't likely in a place for it. That gives her all the control, thats why she is losing it.
You "cheating" won't help your future successes of course. That will take some time to re-heal. Dont rush either as the scab comes off pretty easily.
I am not sure what you can do to get past it except continue talking and figure out why the rule was in place and discuss it. Figure out the root of why she wanted that absolute control and work within that range. Only time will heal what happened.
ps...a month isn't very long, you may have to exercise some patience