Of course good writers can become great writers. I believe that writing is a skill, like any other, the more you write, the better writer you become. Perfection (or as near as perfection as one can get) comes with practice. You can't worry about others' opinions about writing...in all artistic media there are critical people who will undermine what you do. You have to ignore that, you write because you like it and because you want to. Fiction is a matter of self expression and imagination, enjoy it and like what you put on the page, if writing makes you happy never let anyone deter you. There always will be negative people out there, but you have to overlook them and seek the positives.