1) Yes, someone has already thought of your idea. It doesn't mean they plan on executing on the idea. It's the execution, not the idea that matters. It's execution that defines success.

2) Having a partner/co-founder or someone you can bounce ideas off of (besides your bias girlfriend) provides impetus and motivation for someone consumed with the day-to-day of living. If you're having a problem prioritizing your idea along with all of the gotta-eat stuff, then bringing someone in to bounce ideas off of is essential because time is literally fleeting and non replenish-able.

3) Engage other people. Most ideas fail in just this way, because the founder is afraid to share the idea with others. Thinking that person will run off with the idea. The odds of this happening are slim. Most people operate more ethically than that.