Collin, what darknes is asking, is why have you always let her down? In other words, why do you let yourself down and not keep your word?
Look inside and not at what she's saying and don't do things for her, do them for yourself.
These are all valid points. You might need to look inside yourself and figure out the underlying issue that causes you to act this way. I struggled with this one too, and I'm in no way saying this is your answer, but with help from therapists, and counseling, a likely cause for my issue was locked away in my subconsciousness. For years, she would reject the work I did, reject my accomplishments that I worked so hard on, and complain about the way I do things, when in my eyes they were done to perfection. This starts to take a toll on people, and they lose confidence, self-awareness, and start looking in other directions to place their blame. In my case, this is something that I didn't know she was doing, and she probably didn't even know she was doing, but over time, it built up, and built up, and eventually I was drained of what I once thought of myself, as a fully capable, independent, go-getter.
Your case may be different; a lot of the problems we carry over into our adult relationships were actually formed in our childhood. It can be hard to think back of things that could cause this sort of behavior, but practicing replaying childhood memories, teenage memories, adult memories, out loud, can sometimes shine a light on what the underlying causes are.
