Ending a long term relationship is a very traumatic event in life to deal with. You've been with a person that you shared good times and bad with. It's very hard to cut that person out of your life. You've evaluated the situation and really tried to make the relationship work as best you can, but you still are not happy and you don't see any way to solve the situation in a way to keep the relationship in tact.
You know in your heart that this will be best for you and in time, the partner you are leaving behind. Right now you have to be strong and full of resolve. There are three keys to ending a long term relationship. If you are able to follow these, you should be able to end things on a positive note.
Step 1. Be calm and have a plan of action in mind.
Never make a rash decision when you are emotional. Start making your plan for how you'll approach the topic once you are able to think in a level headed way. Plan as much of the process out ahead of time as you can. This will send a powerful message to your partner when they realize this is something you have thought out in advance and not just a emotional reaction.
Step 2. Communicate your feelings and plan with your partner.
Emotion will be present in the conversation, but you must remain calm, almost quiet through the discussion. Many times in conflict, the person who is getting the information for the first time will subconsciously use emotion to try and change the decision being presented. Often yelling and then crying. Be prepared and be strong. This is someone who knows all your secrets and past. It's common for them to play to your weaknesses, in an effort to get you to stay. Many times, this takes the form of guilt trips or in some cases, emotionally abusive behavior.
Step 3. End things on a good note whenever possible.
You had a relationship with this person for a long time. There were strong feelings of affection and love that brought you together once. Keep that in mind and continue to remind your partner of that. Try not to let things get angry or nasty. Present this change as a positive one for both of you and keep a happy demeanor as much as possible as you go through the process of whatever changes need to take place.
It's never fun to end something that's been a big part of your life. If you are at that point and feel strongly enough about it to start down this road, then you are at a crossroads. Congratulations on recognizing and listening to your own feelings. Too many times in today's society, people remain in situations that have deteriorated beyond their control...or so they think. You always have control over your own actions. Good luck!