If your expectation are high accomplishing your goal is only going to make you feel normal if you accomplish your goal and unhappy if you don't it almost eliminates your chance of being happy, when your expectations are low you have more chances to be happy, but there is still a chance you are not going to meet does expectation. From a strategy point of view expecting the worse thing to happen gives you the highest chance to be happy.
If your worst expectations are meet you are going to feel the same maybe a little worse because your body gives you feedback based on how do you perform in the world, and if you are wrong about what did you expected is even better.
There is a myth about pessimist that, they are grumpy all the time, if they would be true pessimist they would either be calm and collected all the time or happy. They are grumpy because they have to high standards and when this standard are constantly unmet this leads to chronic disappointment and and grumpiness.
The philosophy that teaches people that in order to accomplish your goals you have to think positive or visualize yourself as already accomplished. I don't know you but i never accomplished anything by just visualizing or thinking positive, thinks happened when i worked hard and stayed persistent and didn't when i got lazy.
As a matter of fact performing well is making me slack off, i never work harder when i see myself as flawed and deficient, my best work comes out of periods like this. I think this is the best attitude to have for performance.
At the end of the day this emotions have a purpose they are here to help you, why would nature build such a sabotaging emotional system that is making you do worse when you do worse. Pessimism is part of your self-preservation is here to help you to make you resilient enough so you survive even the lowest moments of your life.
Another think i noticed is that pessimism is great for anxiety, think of it if anxiety is a mechanism that it suppose to help you prevent potential threats to your life and well being. If you expect this things to happen to you there is no point in having an emotion that is suppose to make you avoid. Where i live there was recently a pretty big earthquake, as i live in an old apartment building at a high floor, this could be devastating. As soon as i realize that there is an earthquake a wave of panic and anxiety washed over me, when i shifted my attention on the worst cased scenario and i accepted that the anxiety almost disappeared instantaneously even before the earthquake was over.