The best advice I can give to aspiring writers is rewrite, edit and polish. If you have difficulty with grammar and punctuation, find grammar and style books and make a study of improving. If you have difficulty with finishing manuscripts, set definite timeline goals, broken down into weekly or daily word-count tasks, and stick to them. If you write too slowly, concentrate on writing faster by eliminating distractions and save rereading until you've met your goal. Above all, don't buy the idea that your desire to write is a waste of time.