I understand your frustration. I am experiencing the same thing with my girlfriend. She is also a user of said substance. I have been desperately searching the net for answers to this dilemma. It is really beginning to affect me emotionally and psychologically.
Here are two interesting quotes I found on the subject.
1.combination of alcohol and marijuana severely impairs memory, execution of simple tasks, and creates a relaxed feeling resembling apathy ( Lack of emotion or interest.).
2.The few studies that mention marijuana and self-gratification indicate that being high tends to increase masturbatory pleasure because it enhances tactile inputs and fantasy creation. People who like to use cannabis alone also tend to like to have sex alone, with themselves!
I would like to know what the answer is. Much more, how to deal or work with the problem if indeed that is a problem.