A little bit of salt adds flavor, but pour in too much, and it spoils the soup. Similarly, positive thinking in its time, place, and magnitude is a good thing, but too much of it can be bad.
Positive thinking affords you energy, motivation, and initiative. But if you're always Mary Poppins positive, then you ignore reality. The real world doesn't function by your prayers and wishful thinking. Things don't somehow magically get better or go away because you will them to.
You have to deal with life in a more constructive way. If you don't start piling sandbags, the water is going to overflow the levy.
The downside of excessive positive thinking is less about the positivity and more about being ignorant. You have to think about negative consequences. You have to prepare for the best and worst possible outcomes. You have to accept when things are going in the wrong direction. You can't be delusional, overlook the odds, or ignore serious impediments.
You need a more balanced way of looking at things.
A little bit of anxiousness and nervousness jumpstarts you. It encourages you to put forth your best effort because you usually don't get into gear until it looks like you damn well better.
We hear all the time, "Everything will be OK." Why? Because you said so? I'm not going to relax on "it's all going to be OK" when you don't know that or have the power to ensure it. When people tell you that you have nothing to worry about, ignore them. Everything doesn't happen for the best - in fact, a lot of bad things happen that weren't for the best of anything.
So you can have dreams, but be rational, and then work hard to achieve them. The implementation of what you dream requires consistent hard work, determination, and discipline. And realize that sometimes your ability to turn lemons into lemonade is more a statement about YOU than positive thinking.