Can pessimism be better than optimism?
Is there really a benefit to seeing the glass as half empty? I get that lowering your expectations makes for less disappointment and more happiness, but how does one get anything done?

Pessimism does not imply inaction; in fact, pessimism may promote action - think of most activists. Pessimism may also be one of multiple perspectives to analyzing a problem — not unlike wearing the black hat in Edward de Bono’s Six Thinking Hats concept. There, the black hat serves a specific and constructive purpose; just don’t forget to change hats in time. Roger Scruton elaborates on this in `The Uses of Pessimism: And the Danger of False Hope’; a good book review is available here:

It can be more realistic sometimes, though realism doesn’t always mean success.

Pessimism or optimism, it does not matter. The matter is how you can improve yourself base on your characteristics and personality.
People often think that optimism is better than pessimism. Because the optimistic is always happy. They have a lot of beautiful hope about their future. They’re very confident. They know everything will be ok soon. Whereas, the pessimistic seems to be nagative, less confident, doubting or depressed.
Actually, both kinds are equal in opportunity and challenge.
It is said that nothing is impossible. But in real life, there’re a lot of things is impossible. The ability of a person is limited. The optimistic can easily think they can do something beyond their ability but it’s just a illusion. The unexpected failure can makes them never trust themselves any more. It’s hopeless. But the pessimistic is more careful before making any decisions. They define themselves exactly and as a result, the rate of their success is obviously higher.

“Now it is interesting to notice that the extreme optimist and the extreme pessimist agree on at least one point. They both feel that we must sit down and do nothing in the area of race relations. The extreme optimist says, “Do nothing because integration is inevitable.” The extreme pessimist says, “Do nothing because integration is impossible.” But there is a third position that can be taken, namely the realistic position. The realist in this area seeks to combine the truths of two opposites while avoiding the extremes of both. So the realist would agree with the optimist that we have come a long, long way but he would seek to balance that by agreeing with the pessimist that we have a long, long way to go.” - Martin Luther King, Jr.