well, i'll suggest something that can help you with your major problem.
I think the best way to improve your spelling is writing and practising.
For example, write a small story or a small paragraph about anything that comes into your mind, then check your spelling, and rewrite your paragraph with the corrections.
the more you practise, the more you'll get better.
after you reach a certain level that satisfies you, try to move to a higher level of words (longer and more complicated)
this can even help you improve your vocabulary too
Hope this works for you :-)