I am so sorry to hear that you have had attempts after attempts and have not been successful to ctb.
We cannot really advise, because we don't know exactly what went wrong for you in your last attempts. But it seems like you'll probably need a better/more secure rope. Jumping off a cliff also seems like it should have worked, but maybe because of the height or SI?
Regardlessly, I do hope that you find the peace that you deserve though. SN is almost impossible to get in the UK now, but there are many other methods if you look them up on the wiki site.