I'm not sure that's true. All the research I've seen on the subject suggests that successful people in most contexts harbor optimistic rather than accurate views of their chances, skills, and associates.
That said, there's probably a sweet spot.
Optimism changes your chances; changes the risks you will take, changes whether or not you'll employ your skills, changes how you interact with your associates.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
Notes for future OT posters:
1. Please add the 'open_thread' tag.
2. Check if there is an active Open Thread before posting a new one. (Immediately before; refresh the list-of-threads page before posting.)
3. Open Threads should be posted in Discussion, and not Main.
4. Open Threads should start on Monday, and end on Sunday.