I can tell you this from experience- I know where you're coming from, and believe me, don't do this to yourself! My Mom filled out a DNR form when she first went into the hospital before she died. It was painful, but my sister and I followed her wishes, to the horror of my brother, who felt holding onto her was going to provide a miracle that just wasn't going to happen. She had cancer, it was spread all throughout her body and her lungs were filled up with fluid. Absolutely no chance of surviving, and any attempts to keep her alive would have just prolonged the painful, but inevitable death she went through.
Ask yourself this- if you hadn't tried that second round of chemotherapy, would she have been around this long? If you hadn't let her go so she was no longer in pain or suffering, would it comfort you more to see her in that same condition right now? Would she have a better quality of life at this moment if you didn't do either?
Honey, you didn't kill her, and if she could tell you herself right now- she is better off in a better place without any suffering or pain and would thank you for being a great, loving, caring, and nurturing daughter! Many times when we feel helpless, we blame ourselves for not being able to save our loved ones, even though the medical doctors and nurses couldn't do it, either. Please don't beat yourself up over something you had absolutely no control over. One way or the other, she was going to die, and no matter which decision you made, the end result would have been the same.
If you need to talk, PM me. I feel your pain and know what it's like. Just remember, you did what you did out of love and caring, and it's not an easy decision to let loved ones go when we're not ready to part with them.

to you, and my condolences on the loss of your mother. She's in a much better place now, no longer suffering the wrath of an awful disease that takes far too many loved ones away from us.
