Why this is a mistake: Nobody likes to be rejected. Rejection is not quite as bad as betrayal, but it’s still not fun. Yet rejection is an integral part of a writer’s life. You can’t take it personally. And you do need to learn to use it in as positive a way as possible. Most of the time rejection comes as a form letter or slip, so there’s little to be learned. But sometimes you actually get some sort of feedback.
The solution: If you are a writer long enough, you become almost inured to rejection. You learn what part of things you control—the writing—and what parts you don’t. And one part you don’t control is what happens once the manuscript or article leaves your hands. (Of course, to a certain extent you do, in that if you write to the best of your ability you increase your chances of success.)
If you do get that personal letter, try to see what the agent or editor is really trying to say. Understand that these industry professionals rarely write personal rejection letters; they just don’t have the time. So if you do receive such a letter, someone saw something of worth in your writing. Try to find what, and then also try to see where you came up short. Keep this person in mind for the future. You never know where an agent or editor is going to end up years down the line. And publishing is a long-term business. Make sure you mention the personal contact when you query her again.