Sounds to me that the problem isn't motivation, you have a goal and know how to do it. You need more self discipline. There is no way you, or anyone will be able to stay motivated to do something for a long period of time. You will naturally lose motivation. You just need to be able to control yourself.
Also if your school has a track or CC team, join that. It is much harder to skip out on work outs when others are relying on you to be there.