Letter Writer, she will be fine.
Please be honest and forthright when you break up with her. Tell her the truth about how neither of you is truly getting anything out of the relationship at this point. I am fairly sure she feels the same way too somewhere inside of her brain.
@TATAbox I totally agree. I was taken aback when I saw that "advice" and I wasn't sure if it was meant to be some sort of unfunny joke,
LetterWriter, I identify with you in many ways. We are pretty much the same age, both introverted, and we both deal with depression. Please take Polly's advice especially in regards to dealing with the guy because I have been in a similar situation and what you are currently doing will eventually drive you insane. Towards the end of August, a guy who I had been on a couple of dates with posted that he "wasn't into me" on an online profile ( to his credit, he didn't expect me to see the comment). I confronted him on it because I was upset that he wasn't honest with me about his feelings. He and I then decided to be friends. However, any conversation I tried to initiate with him over text didn't go anywhere and he made up some bogus excuse to cancel on me when we were supposed to hang out. So even though he had verbally said that he wanted to be friends, in actuality he didn't care enough about me to invest time in developing a friendship. After a few weeks of this, I sent him a text where I called him out (I may have called him an asshole). Then most importantly, I deleted him from my life. I deleted his phone number, blocked him on Facebook, and deleted his email address. This was incredibly hard, but also incredibly liberating. I don't regret it, because the uncertain nature of our "friendship" was causing me stress. Most importantly, I don't regret it because he didn't care about me so I had nothing to lose and everything to gain from dropping him.
When I initially stopped talking to him, I wondered if something was wrong with me and I became convinced I would never make new friends (I am a transplant to a new city and most of my friends are back home). It's easy to internalize it when people are shitty to you. You start to wonder if it is something about you that makes you deserve to be treated poorly. When I was falling into this destructive line of thinking, I came across an Ask Polly post and, not to be cliche, it changed my entire way of thinking. It was called "I feel bitter about all my exes and I can't get over it" and even though the situation was different, Polly's advice was incredible. She advocated falling in love with all those quirks and oddities that make you unique. That made me assess how I tended to edit myself around new people (to make myself seem less "weird"). Polly said that those who are truly meant for you would appreciate the unedited version of you.
I know that Polly's suggestion to you can seem intimidating and difficult, but ceasing contact with this guy and focusing on yourself is probably the best thing to do at this point. I don't regret doing it for myself.