How do I even explain this...
Let's tell a story. Say, you're playing a video game with a friend, and the battle gets tough. You're both down to your last life. In a clutch, your friend accidentally uses a move that has a 40% chance of killing you instantly, but if it misses it'd kill him instantly. However, the results of the move are still up in the air because of an animation playing that belongs to that move.
A pessimistic viewpoint would be that you expect you'll be the one that ends up dying in the end.
An optimistic viewpoint would be that you expect he'll be the one that ends up dying in the end.
A realistic viewpoint would be that you'd expect the most likely outcome (he dies) with the acknowledging that it can still be the less likely outcome (you die). That, or the odds are so close that you expect both things or don't even know what to expect! Nailbiting ensues...
Of course this is just one example. Most of the time the viewpoint on something depends on the person you ask, the situation that person is in and what the viewpoint is about. And often enough the lines between these three viewpoints can be blurry. It's rarely so cut-and-clear as I described in the example above. Many events also don't have a set chance of happening, there's just a "likely", "maybe", "probably" and "at this rate". Also keep in mind that many events, both good and bad, can mess with our perception of what the chances are of something happening later in life.
Your decision that you don't want to be pessimistic anymore is one event in particular that can change how you view the odds of something happening. Let's not talk about what the odds are on that event permanently changing your viewpoint - that depends on your personality and what you want to do to make that change. It's all up to you now.
I wish you the best of luck. If you ever need to talk to someone, we'll be here for you.