BookCat, after sending my post through I realised that it presumes a character-driven plot - which (as is probably obvious) is what I usually work on. Such an approach is much more difficult to work into anything like a murder mystery or other strictly event driven plot.
Back when I first (outside school) tried to write fiction, I tried to write science-fiction. Since I had a primarily science and technological background it seemed like it would be a good fit ... but it didn't work. I had plenty of plot ideas but nothing kept my interest beyond a few thousand words (and the writing was appalling). If I had been more aware at the time, the first signs of what I could write were showing in those early attempts, but I failed to recognise them and years went past before I made any more serious attempts.
The next couple of paragraphs are personal reactions to your comment: "
within about ten minutes I've developed it into a fully formed plot in my mind". We all work in different ways, so my reactions should
not be taken as criticism of what works for you.
My first reaction was: you lucky sod! Not even my short stories form that quickly.
My second reaction was: no wonder you've lost interest. Any problem, whether it was with my fiction writing or my software writing, that I could solve in ten minutes would quickly bore me to the point of drudgery. I'm one of those perverse people who enjoy difficult puzzles.
As for "
made me feel that finishing a novel is pointless. If this is the case, how can I overcome this?"
It is very difficult to judge your own work, and yet we are our own first critics; we get to decide what is worth keeping and what is not. My judgement is based almost entirely on whether what I have written maintains my interest on re-read. If I can read over what I've been writing and feel myself still getting involved with the characters then I feel I am doing okay and I push on. If I find myself getting bored with my own writing I start to look further back, to see where it is that I start to lose my interest, and that's the point from which I consider I have to rework my story. If I get all the way back to the start then it gets archived as pointless.
So it seems to me that you have a few choices:
a) Take up arjaybe's advice and "buckle down and finish it". For this purpose I suggest looking at this
quote from Stephen Donaldson: "I write every day, and every day I give myself permission to write badly." By reminding yourself that your first draft doesn't have to be brilliant, you can push ahead and actually finish, accepting that it might be crap. Once you are finished you can reassess what you've written and decide what needs fixing.
b) Follow what I do and re-read what you have written so far and try to establish where your mind starts to wander/get-bored. Then see if you can work out why. Do you need more interesting characters, do you need extra characters, do you need more problems or trouble, does it simply need cutting? Could it be that your original plot idea is a bit sparse? (Whether your story is character-driven or event-driven, all starting ideas need to be flexible and adaptable; you never see everything you need to see right at the start.) Related to that is another thing I read from Stephen Donaldson that I found resonated with me: he always needed at least two ideas to form this story around; it was the interaction of these multiple ideas that gave it the extra spark he needed.
c) Get a few others to read what you have, see if you garner enough enthusiasm from them to inspire you to work on it further.
ETA: I do not suggest tossing it aside. If you plan/want to write more stories then I think you should probably force yourself to finish at least one of your open projects before starting on something new. I believe the experience of finishing is an important part of the learning process, and something you need to go through. This is not to say that every story you start must be finished, but you should avoid making a habit of it. So finish one, even if you finish it poorly. It will hopefully give you more confidence in starting anything new.