There's another name for rejection therapy -- it's called sales work. Typically it's the kind of sales work hackers dislike. But plenty of people have built good careers around a sunny tolerance for rejection. Real estate agents, insurance sales people, cold callers, door-to-door sales people -- all of them experience many rejections for each closed sale. One successful sales person told me that "it's just a numbers…
I'm not a "sale type" but I've actually been doing some part-time canvasing lately just to have the 'getting over the rejection hump' experience.
It's easier and more pleasant for me since sales still isn't my job.