You seem to be referring to as what to say when you want to end a relationship with your lover. Remember one thing; you may be breaking a heart when you end a relationship. It is quite easy to break hearts, but very difficult to mend one. One day, when somebody will break your heart, you will be hurt. I would suggest you to think about the break up one more time. If you are really frustrated with the relationship and think you could do much better without him or her, and then go for it. The best way would be to confront face to face instead of breaking up on the phone or writing a letter. Explain each and every thing about him or her that is making you upset and how the relationship is affecting your life. Tell her that you cannot commit yourself. Whatever you say, be honest, because at the end of everything, you will able to salvage a little bit of pride and not be ashamed to look yourself in the mirror.
You can just let this person down easy and tell them some reasons why it's not working out and then tell them that we can still be friends. Then say I love you and give them a nice sweet goodbye kiss on the cheek maybe and leave.