HaleMary I've found writing fanfic very useful for developing my confidence and getting experience in what works, what I struggle with etc... I try ideas, techniques out, refining them before I try them on my original work. It's true that a lot of the work is done for you in terms of characters and backstory, and so now I work very hard on backstory - and it also showed me that to finish, I, personally, need to plan my stories.
I've only been writing fanfic for less than a year but my fanfics are already on some recommended lists alongside writers I really really admire (I was stunned), and someone is currently translating my work. I get some really thoughtful comments by people who actually seem to know what they're talking about. All this keeps me going when I have my frequent Grand Moments of Doubt about my original story.
There is some dreadful fanfic out there, but a few years back (before I started writing again) I started to mentally log everything that makes it bad which I found to be great training. The 'rules' that I worked out that way now turn out to be consistent with the advice in writing books - but I think the fact that I found them myself has helped me to really understand and guard against making the same mistakes - if that makes sense. They feel the same to me as a wrong note does in music now.
There is also some fantastic fan fiction by really good writers out there, so worth a look - the current big site is Archive of Our Own, which has pretty much everything you can imagine. I can recommend some writers if you like.
Sorry for really long post!
I was interested to hear reviews on Scrivener as I was thinking it might be useful but I'd have to upgrade my personal laptop which seems a bit much for a 'hobby'.
My current technique at planning stage involved large sheets of paper (one per chapter, approx), and writing out all the different character arcs and subplots in different colours on different pieces of paper. I then put the key unmovable story points onto sheets, and then worked the other bits around. Sometimes they will make up separate scenes and sometimes I combine various story lines in the same scene. I stapled them to the chapter sheets when happy, although there have been some subsequent changes. Now if I ever sit down and am stuck I look for a new chapter to work on - although I prefer to do it in reasonable order because of all the baggage the characters are carrying.
Books - my favourite has been John Truby's The Anatomy of Story, I also find K M Weiland and Jami Gold's blogs very helpful, even though their own stories don't look like my cup of tea.