A friend in her mid 80s felt the same way but did what she was told - spent her final months having the treatments and died anyway. Her family felt she should have just taken pain medication as needed towards the end but had some quality of life before that point.
You are not alone in your thinking at all.
People have different lives and reasons to push on through, not all think they "wouldn't be dead for quids".
You can only do what you feel right is for you at this time. Do you have family supports ?
You are stronger than you think, though, and this phase will pass !