Many people have similar problems with spelling. Don't get discouraged. There are things you can do.
First- it's important to realize that the spelling in English is quite regular, even though there seems to be a lot of exceptions. When you learn the rules, it really helps. (I will try to find you a good link for this.)
Second- you need to use your other senses to help you. Try saying the letters outloud as you write the words, or singing the letters as you spell. Use your finger and write the words on sandpaper, or trace them on your leg. These activities will engage other senses, and this can be helpful.
Here are some links that have suggestions and resources that might be helpful:
LD OnLine :: Five Guidelines for Learning to Spell and Six Ways to Practice Spelling
Table of Contents
Free Spelling Course
I hope this helps.