Login details not recognised

Almost every time I try to login to iTunes, it comes up saying it's the wrong details. This happens so often I've taken to writing the details down just to make sure I'm entering it right. Each time I have to change it I update where I've written it down. Anyway, after changing it, I go to login again on iTunes and I get the same message!!!!! It's like the changes never actually take effect, even though I've been able to login in to iCloud with them! Any help will be much appreciated. So sick of this, if it keeps happening I think I'll just get rid of the account completely. Thanks in advance.