I have the same problem. What it is is absolutely forcing yourself to get through the song. The more you can finish songs the easier it becomes to get through them with out blocking. It's the same thing with painting, it's a sort of discipline.

Its like forcing yourself to do a 150 hour painting so that your 30 hour paintings are easier to get through. Plus the more you do it the easier it gets. So finishing songs will become easier because you will probably develop a particular tendencies of how you end them. All that will inform your song writing style and eventually it will be natural.

It's just hard forcing yourself to do it because for a while it isn't any fun.