Assuming if you have read relationship websites, you have already firmly come to the conclusion that ending the relationship is the only way? Rather than talking through what might be your issues?
Because the thing is, she might ask why? And if it is out of the blue for her - she probably will.
I only say this as I recently asked why an ex of mine called off our relationship and the reasons he gave me now seem pathetic after 6 years (...however there was another woman waiting in the wings, which you state you don't have)
The sad fact is there are two people in a relationship and it's never going to be happy if one person wants it whilst the other doesn't. Both have to want it for it to work.
And I don't think there is a kind way to do it, least of all if she is not expecting it but once you do it, that has to be it - prepare to go no-contact with this person and move on.
Staying friends to be polite makes moving on a 100 times harder and will probably give her false hope you might return one day? She will possibly be angry and bombard you for the first few days about the hows and whys. Be patient with her but let her know it's over and you're sorry.
Might seem frustrating going over the same things but I feel she would deserve to know, she will be hurting and deserves some closure.
Having said that though, if she does continue, advise her as nicely as you can that she needs to stop torturing herself and there's nothing more you can say.
This is not guaranteed to happen OP, not trying to make you feel rubbish - I just think personally there is a fine line between cutting someone off completely and not giving them information needed to move on and the feelings are often not considered for what the dumpee is experiencing.
And fgs, don't tell her you have to move on with your life and will she one day when she's ready - we don't wanna hear that waffle yet haha