oh gosh, i'm probably not the person to ask because i'm really erratic with my writing methods! like sometimes i can only write if i've really planned it out beforehand but sometimes i can't write at all if i'm restricted with a plan, sometimes i need music and ambient noise (i do a lot of writing in starbucks) but sometimes i need the dead silence of 2am and a dark room. sometimes it'll take weeks for me to finish something and sometimes i can churn out 5k words in a single morning. like seriously, i'm all over the place with it. 

i guess the only thing i can say is don't give up with it? if you think it's not working then leave it and take a break for a few hours or even a few days and then when you come back to it, it won't seem so bad. and also, if it's fanfiction, make sure you really know the characters well. readers will notice if something is OOC and there's no good reason for it. do your research, basically. um... i'm a big believer in rewrites and drafting. if you don't like something, scrap it and write it again. 

i don't know what else there is, really. writing is one of those things that's so personal. you just have to find the ideal conditions for you, whether that's a coffee shop or a park bench or just your bedroom. and don't force something because you'll probably hate it.