Dang man, that sounds rough. Being that I am a cold, emotionless
robot lich, I cannot say that I've ever had the misfortune of associating something like that closely with another person, other than maybe associating my crippling League of Legends habit of last year with my then-roommate and the other guys in my dorm, most of whom have since gone on a different path than I have (although, my reasons for not getting back on that particular horse have more to do with me wanting to do well in school, rather than me being only able to play with those people). It's not the same, and it's probably not even near the same level of what you feel, but know that you have my sympathies.
Of course, you could always go the spiteful route and find a new lady to play DS3 co-op with all while subconsciously cursing your ex with all of the fury of a thousand suns. That's what I'd do, but I'm also young and immature and recognize that doing so is probably not healthy.