Well, it sounds like you're ready to end the relationship rather than help her to improve on what she's doing wrong (just based on the wording of the post), so I don't think there is really any way that it will end without hurting her feelings, but just leaving out accusatory words and using phrases such as "I feel" and such are always recommended for discussion.

But honestly, I think when you end things it is much better to leave out words like "horrible" and such, because that's definitely a way to break someone's confidence. Maybe one day with guidance she will improve, or even what she does now might be better suited for someone else, but if you break her confidence all together, she may never even try with someone again.

That's the best advice I can really give, without any real back story.