First, don't start a thread about your self-pity or you will just get lots of negative posts and people laughing at you. Which is not good. It's up to you on whether you want to stay on the forums or not and nobody else. It's not a big deal and there is more to life than posting on here all day everyday.
Secondly, as you write more stories you will improve with time. Nobody writes their first story and it suddenly becomes a masterpeice. You will get criticism, everyone does. Even bestseller books get criticism as they don't suit their taste. Just perfect your writing skills, experiment a little until you find one that works for you and people like it. If your looking for help on how to write then don't take no stupid courses or read manuals, instead learn from other sources. Read some books and observe how they write and what language etc they use. Read some of the current popular fan-fics here on the forums and learn from them.
Thirdly, when you do post a fan-fic you will get criticism. Some constructive, some not. Don't listen to the 'Grammar nazis' who nitpick at everything and try and put you down. Everyone does make mistakes and these little things will improve with time. Also don't listen to the asses who just post pointlessy and say its rubbish. They most likely don't have a clue about writing so just ignore them.
I will be looking forward to reading your future Fan-Fics anyways, Good Luck
EDIT: Also as some people have said why not join some RPs. Most people on their are light about rules on perfect grammar and writing so it should be good practice on improving your skills. Alongside that it is fun too!