If you don't want to go out with her, then just dump her. Nothing hard about that, no excuses. Anything else is simply relative, but ultimately shouldn't effect your decision. Whether she has issues or is a nice person, fuck it. There are plenty of issues in the world, pull the trigger.
She needs a friend when you two break it off. It's hard when the person you care about just shoves you to the side as if nothing happened between the two of you. As I said before, comfort her and be her friend. Let her know you are friends, but the romance has ended (no matter how great it was at times). Trust me, if my ex had been more supportive after all the crap we went through, I wouldn't hurt as much as I do.
^I agree with this on principle. However, do be careful in your friendship. You don't want her to hold on to false hopes like (oh we're friends now, maybe someday he'll...). Be firm about being friends and nothing more. Obviously you're not going to intentionally lead her on, but you might end up doing that unintentionally if you're not careful.
I'll answer both your posts in one. To this one; You claim she is your only friend and best friend, and you have been with her for years against what you say are your wishes... Not too bright... and not very emotionally mature of you... you admit to having a co-dependent relationship with her, last time I checked, that isn't a very emotionally healthy way to live... To the other post... She is never going to see it? How do you know that? Do you not allow her to use a computer and keep her from going out where she may access one, or just monitor where she goes when she is online? What happens if you are wrong and she does see it. Is she going to be happy when she reads what you have said to the whole world about her, regardless of whether or not they know her? According to you, her emotional state isn't that great to start with, and here is the guy she is living with and to her belief loves her saying she is fat and stupid (no those aren't the words you used, but they are the ones she will see if she reads it)... But hey man... what the hell do I know...
Like others have said. The longer you stay in a unhappy relationship, not only will it make it harder for you to end it...but will also be harder for and unfair to the other person. You have to be true to yourself before you can ever be true to anyone else.