Sounds like you just need to take a break and do no quilting for at least a week. When you're hands are itching to get on the machine and start piecing, then you will be able to pick a project.
If that doesn't work, put the name of each project in a small bowl and pick one name out. That's the project to do. If you pick one out and say to yourself, "But I really wanted to do <name project>." Then make that you're second project.
Hope this helps.