I like being realistic at all times. Being positive and optimistic can make you rush into something without that much of a thought. Somewhat clouding your thought with "this is going to be good","it's going to work",etc...
Same goes for being negative and pessimistic. Not being able to see something or even maybe denying to see due to being, well negative.
Keep it realistic, look at what's going on, follow the market and play smart. Don't rush into things, that's my best advice.
This is not about being blind and rushing into things. This is about being in a state where you can make decisions. Compare that to a state in which you can only sit and wait. In other words, you can be positive while remaining a realist and overall cold-blooded when you make your trading decisions. I see no conflict or contradiction here. In fact, being realistic already assumes having a positive attitude.