Struggling may have a significant impact on how you grow, but there's really no need to go out an look for for a reason to struggle. There's enough difficulty around every corner and I seriously doubt that anyone would intentionaly dump girlfriends to make life more of a struggle. Even in the name of art.
People may be selfish, but its very rare that anyone is intentionally cruel. Maybe ignorant, or unaware of the pain they are inflicitng. So I doubt you act the way you do out of cruelty. If you're constantly repeating the same pattern and causing yourself and others harm there is a lesson there that you haven't learnt yet. It's that simple.
Fumbling, It seems that your current reasoning on relationships has been floating around in your mind for quite a while. But you're still suffering... so there must be a flaw in your reasoning. Contrary to popular belief suffering is not that natural state for humans, or any other creature. Suffering is there to alert you to something that is wrong with your body, or wrong with your emotional state, or perhaps something that is wrong in the way you live your life.