This seems weird, but I found an article about the difference between a "lover" and a significant other, friend, soulmate etc... and I felt like our relationship was like this. Not in terms of romance, but in the way that she only ever wanted me after a while to do things for her, like be her photography model, or help her avoid an enemy, or get over a heartbreak. When she didn't need me, she didn't want me around just for my company, and I'd be left wondering why.
Early in the friendship we were very close and I think enjoyed eachother's company mutually, but later on, especially into my depression, she began to drop me. This is the article, and I really think it's well done:
http://www.buzzfeed.com/mattortile/the-one-who-waits

In a way it's perfect, because the author wants a friend in a soulmate, just as I wanted a close friend, not a user.