Breaking up with someone you have been living with is as traumatic as a divorce, so read as much about divorce as you can. It will be traumatic and heart-breaking and you my wonder if you have done the right thing, but stick with your decision. Sit your partner down (if he is not abusive - otherwise take someone with you) and tell him it is the end in no uncertain terms. If you decide to end it, make it a clean break for both your sakes, so you can move on and heal.
While breakups are seldom easy, sometimes they are absolutely necessary for your wellbeing and the health of your former partner. Begin by writing a list of the relationships assets and deficits. An extensive list of deficits is a strong indicator that it may be time to move on with your life. Remember, relationships are only as healthy as you are. If your boyfriend is physically and/or emotionally abusive, forgo the list and seek safety and good counsel. Anyone who threatens that the end of the relationship will cause trouble or pain for you is not worthy of your love and respect.