It would seem quite pessimistic and anxiety-inducing to always assume the worst. However, there are benefits to consider here. When approaching an uncertain situation, it may be wise to assume the worst scenario. This way your expectations are low allowing for a higher probability that you’ll be pleasantly surprised. You can also make assuming the worst a more proactive measure than this though. Rather than just having low expectations to bolster your pleasure of something you can act on those expectations to ensure your pleased.
It’s important when assuming the worst to not act on that assumption the wrong way. The wrong action would be to detach from the worst by cancelling the engagement or by becoming discouraged or distraught. The right action would be to prepare for the worst with a mentality of believing you are entering a low quality, disadvantageous, and bloody poor situation. Assuming this to be the case, you should thus be motivated to prepare preventive and counter-measures for your imminent failure. The measures taken can be things such as researching and practicing for different problems, forming back-up plans, containment protocols and disaster recovery methods, reinforcing weak spots, or adding more elements that can facilitate success. Doing these apparently pessimistic things will give you optimism that bad things can be neutralized or made good, as well as allowing good conditions easier to make great.
Thus, when done with proaction in mind, assuming the worst is not pessimistic so much as it’s optimistic. Preparing for the worst is synonymous with preparing for success. If you prepare yourself for the worst possible situation then you are ensuring a better or best situation. In other words, you are planning to achieve a great outcome by preparing for a terrible one. That sounds pretty optimistic to me, regardless of the paradox.
Perhaps assuming the worst is the fork in the road leading to optimism and pessimism. Assuming the worst and letting it break your spirit will lead to the realm of pessimism. Assuming the worst and letting it inspire your proactivity will lead to the realm of optimism. Whatever you decide to do: please, assume the worst responsibly.