So you're pissed because she's more focused on the American Idol thing than on you at the moment. My advice: get over it.
Relationships, as already stated, are about compromise. Give and take. It takes two people to make a relationship; that's two seperate minds, two seperate egos, two seperate sets of values. When one person controls the relationship, a break-up is inevitable. She's actually doing the smart thing in letting your comments slide off her back. Not trying to judge you or anything, but if all I ever heard from someone was how bad they want to put a gun in their mouth, I wouldn't want to hear it, either. If you're having problems, you've got to find a healthier way of expressing it than saying you're going to blow your own brains out. Nobody's going to take you seriously, particularly if they're used to hearing you say it.
As for this American Idol thing, regardless of how much bullshit you think it is, it's important to her. All you're going to do is create a distance between the two of you by talking down about this dream she's trying to pursue.
Now, I know it sounds like I'm ragging on you, but I'm not. You just need to understand that there will be times in a relationship when one of you wants to put your own needs first. If you support her during those times, she'll do the same for you.
Also, knock off all that crap about wanting to shoot yourself. Say it once, people listen; say it a dozen times, you're all talk and they'll just ignore it.