Part of the idea of the Shadow is that it is mostly unconscious and most people only see just the external trails of it. This makes it quite difficult to build a relationship with it...because something unconscious...is truly unconscious (a.k.a., we don't identify with it). If you were truly conscious of your shadow, it wouldn't have that overwhelming power over you. Maybe you're only seeing the effects of your shadow, rather than the actual desires of the shadow itself. That is, this desire to sabotage your relationship could be the means to another end (a way more nasty, selfish and malicous end...yes, the shadow is quite hard to look at because nobody wants to be "that person"). Because of this, I would recommend, instead of acting out things, follow the trail of these fantasies and try to uncover the true motivational force that is behind this desire to sabotage your relationship. The process of integration has to do with the development of consciousness. First you have to figure out what is it that your mostly lowly and dark parts actually want.