It is harder then heck to do. The first thing is you have to realize your hurting this person more the longer you stay with them, not to mention yourself. The sooner the "band-aid" is ripped of the sooner you can heal and move on and the same goes for your soon to be ex. significant other. I can't guarantee the wound will ever heal completely for you or them, but hopefully you both can take something away from this as a lesson for the next time. Be strong.
Take A Sheet Of Paper. Make Two Lists. On One Write All Of The Ways This Person Is Good For You. On The Other Write The Negatives. Compare The Two Keeping In Mind Specific Item LISTED As Opposed TO The TOTAL Numb- er:) :(