A pessimist is a person who expects the worst and that things are bad, and tend to become worse. There is an advantage to being a pessimist. If things go wrong then they were right. If it does go right then it was luck. As plans tend to never work out exactly and new ventures tend to fail 95% of the time; so the odds are in their favor. They have ample ammunition for their thinking and will rarely take on new challenging goals.
An optimist is a person who is disposed to take a favorable view of things. The advantage to an optimist of this attitude is believing that given time, things will work out in the end. They will take on new and challenging goals without full consideration of the risks involved. They will pay little attention as to why it failed and will tend to repeat their errors.
We need a balance between the two viewpoints when looking at our lives. We need to tread the middle way and use both mindsets in formulating our plans. Start with an optimistic plan then look at with pessimist 's viewpoint to find flaws and holes. Test and research to repair the plan.
Remember that plans are maps only and we will encounter unanticipated events that will require flexibility as we progress to our targets. This approach would more likely produce a valid plan of action.