When you get bored with a project - it will die a slow and painful death.
When a project dies a slow and painful death you will be discouraged because you failed.
When you become discouraged because you failed you lose confidence as a writer.
When you lose confidence as a writer very bad things happen.
Don't get bored with a project.
How do you avoid getting bored with a project? That's a good question. I don't know. What I do is as soon as I feel I am getting bored with a project - I shelve it. No project - no matter how exciting at the beginning - is worth losing confidence over. So I shelve it and move on.
Yes this means I can burn through a dozen projects in a few months. But in order for me to be successful as a writer I need to be interested in the project. This is probably a weakness I have as a writer but I am okay with that.
Embrace your weakness and either overcome them, or work with them. Turn them into part of your method.