I know what you mean. I have a similar problem myself. People like us have a major flaw in our personality, we get bored way too fast.
We get all excited when we have a new idea or receive a new project and put ourselves into it, but then all that energy get burned and we dump the project, usually without even realizing we did so.
Many times we dump a project because we get all excited about something new and the cycle starts again.
I wish I could tell you, and I myself, that there is a simple solution to this problem, but there isn't.
You just need to stick to a current project and don't allow yourself to get dragged into a new one.
Find reasons to complete the current project, try to remember why you even started working on it in the first place.
Another thing I suggest you should do, is list your goals. Define completion.
And I suggest you leave previous projects alone, because there is no point of fighting the past, stick with the present.