This is why I don't share really good ideas.

You can say that ideas are worthless, but when you put a good idea in the head of someone with more time, drive, or resources than you, you will create competition (and possibly put yourself out of business before you start).

My last couple of business ideas (which now make me good money) came from someone blabbing about it on an Internet forum. My idea was a little different of course, but it did directly compete.

Execution is everything. An idea evolved and the seed of the idea may be the same but the end result will almost always be different. The peer is a dick for such a move but it's not something to dwell on. If you are to succeed, it will be because of how you execute or lack thereof, not because someone stole your seed idea, generally speaking