I feel so trapped

I feel so trapped. My girlfriend and I have been dating for 3 years now. I don't feel for her the way I used to, especially when she's not around. But she's my first real girlfriend, and I can't imagine life without her. She's pretty, and she's good to me, and when I'm around her I can't help but soften up. But when she's not around, all I can think about is how I'd rather be with someone else and I think it's time to move on. But she loves me, and breaking up with her would break her heart. I know because I've tried, and she cried like I've never seen anyone cry. It felt terrible seeing her that way. Knowing she cared about us so much made me feel better and I took her back. But now I'm wondering if that was a good idea. She's such a good person but she's sick all the time, which sucks because it makes me less attracted to her but at the same time I want to be there for her and help her. I encourage her to make healthier choices but she just gets defensive and doesn't listen. She was born with health defects so there's not much she can do about a lot of it. And because of this, along with a rough upbringing from ultra-religious parents, she's sad all the time. And this may make me sound like an asshole, but it's draining. I don't know how much longer I can deal with it. I try to help her but she won't have any of it. She wants to stay in her comfort zone of negativity. Her job involves social work and yet she doesn't seem to grasp how her attitude is affecting her life. She's stubborn. And I probably would be too if I were in her situation, I can't imagine being in her shoes with the shit she's had to go through. But I want to be her boyfriend, not just her caretaker. Writing this down is making me feel selfish, but I'm 26, this is the prime of my life, even if she was happier and healthier I'd still be curious as to what else is out there, like I said she's my first real girlfriend, and I've only had one fling before that, and that's it because I'm usually very shy. I fantasize about other women all the time. I'm not happy in this relationship, and she doesn't seem happy either, even though she tells me I make her happy. If that's true, I must be one of the only things that does. So, I don't have the heart to telll her any of this. I feel so spoiled and selfish, and undeserving of anything better, but at the same time, the fact that she doesn't make me feel like I deserve better makes this seem even more like an unhealthy relationship. And I feel so bad for her, but pitty is not something to base a relationship on either. But I'm so scared of being alone, even though the rational part of brain knows that I wouldn't be alone forever and I'd probably be happier with someone else. I don't know what to do. We're not married, we don't have kids, I haven't cheated on her, I need to get out while I still can before I really hurt her, but I don't have the balls! What do I do?? How do I get out of this without devastating her? Is this a unique situation? How do other people deal with this kind of thing?