There's not really any trick to it. You just pick one and build it anyways. At some point you will think it's a waste of time and useless. Just keep going, the hardest part is pretending to be jazzed about it when discussing it with others, but it's essential that you keep pretending it's awesome until you are excited about it again (or it's done, whatever comes first).

I think the key problem statement in your post is "I will usually invest several hours into each idea". You really need to put in several hundred hours before you can tell if something is worth it or not. The "excited" phase is really just a tiny fleeting moment in a project's lifespan.

In reality, projects are rarely clear flops or wild successes. People love describing them that way because it makes a great story, but it's a myth.