I had same problem. Think about it this way: You are not generating new task to do, you are just identifying existing. After you have identified them you can choose what to do. Do you want to handle things when they show up or when they blow up?
If you haven't identified all things, you probably let external situation do the choosing for you (latest and loudest). I understand it is not easy. But think what's it like if you don't clarify all tasks: then you are like a ostrich, burying your head to sand, thinking "if I don't see it, it cannot harm me".
After you learn to commit to those things you really find important for yourself, it is much easier to say no to extra stuff.
By the way: remember to use someday/maybe list. At least for me, many projects end up to it, because I'm really not ready to do those thing now or I don't want to do them yet.
Total GTD newbie, better with the GTD theory than implementation.