Unless you have reason to think she’ll go psycho on you there is nothing to fear by simply telling her that you want to move on.
Try your best to make the conversation about what YOU want and not about her shortcomings. Don’t let the conversation deteriorate into a drama filled crying, begging, or bitch session. Keep the conversation under 30 minutes.
It is not your responsibility to help her cope with your decision. That is the role of her family and friends.
Do not confuse her or send her mix signals by contacting her under the (guise of friendship) or reminiscing about past events. It's probably a good idea not to accept her calls or respond to her emails, texts, or letters for at least 6 months. You don't want to give her "false hope".
If you have truly decided the relationship is over then behave as though it’s over at all times! She needs to be able to move on with her life. Staying in contact with her too soon after a breakup keeps her from doing so. It's not fair.