When I first started writing about 12 or so years ago I had this same issue. I'd start novel after novel (20 to be exact) and about 10k in I'd be ready for something new and the next best idea would take over. It was infuriating. Finally I took a workshop that got me into writing short stories -- most of which ended up being about 5-10k. It helped, but I still had my heart set on novels and I kept coming back, but still couldn't finish. For me, the problem was a wealth of ideas and an inability to focus because of it.

Long story short, I read a book called "365 plays, 365 days" or something like that by Susan-Lori Parks, and it made me realize that maybe I needed to spend some time doing short stuff first to get the ideas out, and to get into the routine of writing before I could really sit down and dig into the longer stuff. So I set a goal for myself and did a short-short story a day for a year -- it was a mess and most were not well thought out -- but it let me get out all sorts of pieces of ideas and it let me experiment with my writing. The biggest thing of all, though, is that it got me into the routine of writing consistently every day. When I finished, I was able to sit down and really focus on writing my novel, and now I've finished two, and I found my skills had vastly improved from all the experimenting. The 365 thing is very ambitious, but I'd definitely recommend trying short stories for a while and see what happens. Or even maybe shoot for a novella. Sometimes starting short and practicing finishing is all it takes to build up to a full-length novel.

Tl;dr - Try doing some short stuff for practice and to get some of your various ideas out, and then settle down on working on one project if you want.