If you hold an opinion, no matter what it is, no matter what subject it's on, then the odds are that it's wrong. And although that can smack of arrogance from the one telling you this, it's really nothing more than a statistical observation. Most scientific hypotheses turn out to be wrong, most new businesses fail within the first few years, most romantic engagements end in breakup or divorce. Yet we've grown used to these depressing figures and we carry on because we're buoyed by frequent-enough success. We learn at an early age that failure is normal and necessary and we all have the strength to try again. Some call it practice.
Critical thinking means being critical of your own thoughts, and it begins with believing that your first opinions are probably wrong, but that there is a method to finding the ones that are--if not completely right--at least on the right track. It is very easy to practice, and the habits you'll develop can amplify the intelligence you already have.
Imagine yourself coming nose-to-nose with a huge object, and feeling the instinct to step back and get a good look at what it is. Is it a rock? An elephant? A tree? Your first impression is going to be limited because you weren't expecting to run into anything, and you're too close to make out the big picture. This is the habit to practice when something sudden and unexpected happens in your life, such as getting shorted, contradicted, cut-off when driving and so-on. Each time this happens, something else may be part of the bigger picture.