Figure out why you lost it. If you don't know why the relationship ended, chances are it will end for the same reason again. So if you have any confusion or doubt at all, find out for sure exactly what the problem is. Maybe you already know.
Take responsibility. This is difficult for most people. Can you do it? Own up and tell them, "I was wrong. It's my fault." Now say it again, this time with conviction. I know you want to place some blame on the other half of this relationship. Remember, it takes two. No one is perfect, and your significant other probably messed up too. But if you are really trying to win back the love you've lost, then you better quit pointing fingers and start taking responsibility.
Apologize sincerely and specifically. Now that you've gotten over that taking responsibility hurdle, this step won't be quite as difficult. Practice saying, "I am so sorry for __. Please, will you forgive me?" Fill in the blank with the specific action(s) that you have done that contributed to the break-up. Don't be tempted to leave off that last part. Asking for forgiveness is very important, as it puts the ball in the other person's court and lets them know that you are serious and that you are willing to be humble and seek forgiveness.
Commit to changing. Whatever you did before, you have to make sure you don't do it again. Not now, not ever. So figure out what you need to change in your life, habits, work, social life, friendships, family, style, spending or whatever. Take drastic action if needed. Start making these changes, and tell the one you love about what you are doing. Don't be surprised if you get a critical or disbelieving response. That's okay. Over time you will prove through your actions that you are serious, and that negativity will change.
Get a system in place to hold you to those changes. To make sure you do prove through your actions that you are serious, don't give yourself a way to back out of the changes you have made. Get a good friend or family member to hold you accountable. Make it so that there is no going back.
Ask how you can rebuild trust, then do it. Love is built on trust. Without trust, there can be no relationship. Ask the one you love what you need to do to earn back trust as a friend. Don't push too hard here. Whatever suggestions are given, take them seriously and do everything you can to build that trust again.
Start a sincere courtship. Once you've started rebuilding trust, you can start rebuilding affection. Go slowly at first. If you are rebuffed, back off. Be respectful. Be creative. You can find ways to show your sincere, unchanging affection simply to giving undivided attention, by being available, attentive, and helpful. Read the article "42 Ways to Court Your Love" in the resources section.
Be patient and consistent. Building trust and gaining affection takes time. Winning back love after a break-up, separation, or other conflict in a relationship takes time. You can prove, day by day, that you are different and able to provide what is needed in the relationship.