Hiya, don't feel too bad about it >_< I procrastinate plenty of times and that is why I barely get any work finished (guilty as charged). What I do is just force myself to write nonsense until I get inspiration, and then, I reedit what I wrote again lol. That is just a method that works for me though.
It does not mean that you are a bad writer if you cannot write long fics; it just means your attention span is kinda short (and in my opinion, long fics are harder because you have to ensure the plot and characters are consistent, but honestly, how does one remember all the events if the fic is soooo long?) Anyway, if you really want to write long fics, I suggest that you start small. From a one-shot, maybe you can then write a few short chaptered stories and then proceed to longer length fics. Also, you can read stories related to the genres/ideas of your fic, but most importantly, I think it is important you have passion for the plot and characters at the first place so that you will be determined to finish it no matter what.
Regardless, good luck with your fic! I'd say don't worry yourself too much because writing is supposed to be a fun thing. Just relax and see how things go ^^