Planning is hard. I got me a programme a few years ago that helps me keep track of things even if I am more the ‘write as I go along’ kind of writer as opposed to the ‘plan it all out before I write anything’ approach. It’s called the Snowflake method and the programme Snowflake Pro. Basically allows you to keep tabs on characters and the persona and motivations and so on, but also write concept and a lot of other things. I use it my own way, which is what the author recommends any way.
Finishing, I’m right there with you. Not completely certain as to why - perhaps I have a short attention… oh, look, shiny thing!
…where was I? Oh yes, short attention span. Or maybe I get bored quickly. I have no clue. I do know I have way too many ideas fighting over attention and I need to regulate them.
I simply try to persevere. (easier said than done for me, ha)
For some, having a deadline helps. For me, it stresses me out more than help.
Others are helped by, say, posting there WIP here. For others it can mean the end of their project, for many reasons. Bad feedback, for one thing. Bad as in non-constructive. Seen that happen before I signed up here. Depends on you and who you get posting.
I’m still figuring out what works best for me, different things work for others but not me, so my quest is still on-going.
The ‘Snowflake guy’ have approaches that might help you with this too, and that part is free unlike the programme, just an on-going newsletter.
Hope you find something that helps!