Spelling problems can be caused by several factors, one of which is poor visual memory. Some people can't remember the sequence of the letters in a word. The letters seem to "jump around," making it hard for the visual centers in the brain to "take a picture" of the way they were organized. When you try to write a word, you may spell it the way it sounds. If you make a misteak, er mistake...the brain doesn't say, "Hey, that's not the way it looks!" since it has a distorted picture of the word in it's visual memory. Some kids study their spelling words all week long, and because they've practiced them so much, they do pretty well on the end of the week test. But ask them to use those words later on in writing (an e-mail, for example) the memory has faded and it's back to spellcheck.
For some folks, the attempt at a word is so far off the mark that even a spellcheck's "brain" can't recognize it. This is especially true for people who have what's called poor sound/symbol association. Their brains don't "hear" the sound that comes into the ears in the correct way, so the sound isn't stored correctly. Then the next time someone is asked to spell a word, the brain can't match the correct sound to the correct letter. So, "pit" becomes "pet" or "nature" becomes "natcher". And you can forget about those double vowels like "ou" or "ie"!
Adults with these kinds of spelling errors can rely on spellcheck features. Writers who can dictate stories or letters can be helped by speech-to-text software that type what you say. It's very good at spelling, too! (Try Dragon NaturallySpeaking.) Some kids with these kinds of spelling (and related reading) difficulties, which are called phonemic awareness problems, are helped by a program called Fast ForWord from ScientificLearning, which purports to "retrain" the auditory centers of the brain to process the subtle differences between similar sounding letters.
Know that you are not alone, and there is help available. And whatever you do, don't let this spelling problem hold you back.