Remember that formula and you will be fine.
It's not a case for lowering your expectations, but it does set a framework for approaching different endeavours. It forces you to ask the question, if this goes amazingly well, how good is that really? Furthermore, it forces you to ask whether you are putting too much energy into your expectations. Finally, it's why slot machines (based on unexpected and uneven payouts) work so well. :)
Basically, the level of stress in a population or experienced by an individual has little to no correlation with happiness. The type of stress, especially the more insidious one, is probably more important.
Then, I'll add that it's all about your expectation. If we hope to put food on the table everyday, and we succeed most of the time, we will be proud and happy most of the time. If we expect to strike millions by being an entrepreneur, because we all know that we're nobody if we're not an entrepreneur, we will endure a roller-coaster of deception and small victories, with no quantifiable bearing on our success or lack thereof...
"let Happiness = reality - expectations"
But it’s obviously an oversimplification
If "reality" means competing against other smart/driven humans, be prepared to lower your expectations.
I would say, come to know this and you will be fine: Happiness = your natural state
For example, in the winter olympics, there was a world class ski player that had been a champion for the world cups but not an olympic medalist. She already had been in positions like 7th, 6th, 5th, 4th and she just wanted nothing but a medal. In the Vancouver olympics, she went to the final round and was very close to winning a bronze medal, but finished in 4th place. She bursted into tears and clearly seemed very unhappy in the interview.
In the opposite case, if reality is far lower than expectations, like you couldn't even pass the qualification round, you can think that like an accident. I have seen this in the PyeongChang olympics that some good players not even passing the qualification round because of the windy weather. They clearly looked unhappy, but not bursting into tears, because blaming the wind will relax their unhappiness.
Many well known Buddhists talk about not being dogmatic in the face of your mental activity, but merely taking a step back and asking yourself: Does this serve me?
We could measure a decrease in the happiness of a suddenly very driven and successful engineer alongside the increase in their income, take that surplus income, and then convert it into happiness for others by way of purchasing critical medicines for impoverished people.
Summed up, it's possible for it to be good for certain people to decrease their own happiness to increase their outputted value, and then leverage that increase in value to provide much more happiness for many more than 1 other.
One perspective might be defining progress to be maximising the expected long-term survival (that is, non-extinction) of our species. This framework could justify any amount of suffering of individuals - what's good for the population isn't necessarily good for individuals, and vice versa. https://nickbostrom.com/existential/risks.html
If happiness is subjective, perhaps some individuals are naturally capable of greater levels of happiness than others -- https://www.smbc-comics.com/comic/2012-04-03
Another perspective might be to get a bit carried away with using accounting heuristics when trading off happiness now versus happiness in the future, and decide that a 4% annual happiness discount rate makes sense, so 100 units of happiness now is better than 2 units of happiness in 100 years time (this kind of valuation might be consistent with the human endeavour's response to the threat of climate change).
If what you're doing would potentially create more "happiness," or "positive outcome" for others.
But most of what most of anyone does has no significant or lasting impact. "Merely satisfactory" gets the truck to the dock on time, the Uber customer picked up and delivered, or the HN post replied to.
Aiming at excellence is great, but in most cases pushing toward excellence at all personal cost, successful or not, doesn't really move the needle.
Take care of yourself first, unless you're an extraordinary individual in extraordinary circumstances.
Most of "this" just doesn't matter.
Many successful people use positive thinking to help them succeed in challenging situations. If you lower your expectation, then you will never become successful.
I think having expectation oftentimes create psychological disorders what Logotherapy identifies as anticipatory anxiety and hyper-intention...
I think key is to actually NOT to have expectation. Expectation are result focused. What's important is to be process focused. And process is more about thinking positively on each step. Or better yet... use Paradoxical intention to face our worst fears.