Why are some people better at letting go than others?
I'm not a psychologist, but my amateur perception from my own life experience is that it has to do with expectations for the future, and is related to optimism/pessimism.
Many things in life are experienced only one at a time - relationships, books you're reading, places you're living, jobs you have, etc. To use the job analogy, imagine that you leave college and take the first job you're offered. It turns out that you love the job, and you know that you're the happiest at work of all your friends, but it's just a little short of your ideal job. Perhaps you'd like to earn a little more money, or work in the same role in a more interesting industry.
Somebody, like yourself I suspect, will think "wow, this is a really great job, I couldn't possibly leave it, because what if I can't find something else as good?"
Somebody else, perhaps the person you're striving to be, will think "wow, I just took this job because it was offered, and it's great! Imagine how great a job I can find if I embark on a conscious search and really try to get the job of my dreams?"
Sometimes you have to give up the "pretty good", to make room for the "awesome". I suspect you're not willing to risk the "pretty good", because you're worried that you won't find anything above "OK".
You have to get comfortable with giving up the "pretty good". (I'm not suggesting that everybody should quit "pretty good" jobs immediately; I'm referring to people who are in a position to move on - financially, etc - but find that fear is the primary factor holding them back.) Accept that the next one might only be "OK", but that there are still other jobs, books, and life partners out there for you to find, and you can keep going until you find the awesome.
How comfortable you are with saying goodbye to the "pretty good" is related to the extent to which you trust that the "awesome" is out there, and that you can find it.