I was in the same boat for decades, RightersBlock. I collected a large Tupperware box full of scraps of paper with notes and ideas on them and resolutely failed to take them anywhere. Every time I got a bit of energy to 'start' I'd last a few days then it all break down and collapse. I couldn't plot and I was ludicrously embarrassed by my characters a day after they had 'sprung into life'. But I would think about the ideas and the scenes all the time.
It's a complicated issue but I found a number of things switched me on to writing.
Firstly the ideas - I won't repeat all the sage advice and thoughts given above, (I too strive for originality, but now recognise Anya's comment on it being a two-edged sword with regards to tackling publishers). However there will be a point when reading other peoples work where, how shall I put it, you start going "Well, this is great and I'm enjoying this immensely but I see exactly what happening here plot-wise and actually I'd take it this way..." i.e. the edge is taken off (even a great) book because well you've read a lot and your ego is starting to itch to try and write something better.
--> So keep reading. Preferably Loads and lots of different genres, I found that helped a lot.
Secondly finding your habit - how was I to know that I love writing at about 5-6am in the morning, fresh and awake just sleep and shower, pink skies as the sun rises and mist on the ground? (Also even London is actually quite quiet at that time - it's a lovely still time.) However there can be other things that might be blocks i.e. for some reason I found that I can't write regularly unless it's on a computer with a keyboard, the thought of me scribbling out a novel by hand feels me with dread (as a left-handed doctor of physics cum software engineer cum Investment banker my handwriting is now as hard to decipher as cuneiform - and that's just for me myself
)
--> Habits are important for you to find the time, because you are going to need lots of it and getting into a groove really helps. This really depends on your circumstances - but the good thing about trying to find this is that finding good timeslots doesn't cost - just needs a bit of willpower.
Finally there was the 'hook' that was the final push for me. The thing here is it's different for everyone. Perhaps you need to find that 'How to...' book that speaks to you clearly and gets you started. Perhaps you find a mentor somewhere that can push you into finding how to structure your thoughts. So when I feel a bit jaded just reading other writers in websites such as Chrons helps to reinvigorate me. But to start everything off I personally needed a bit of structure - I took a dirt cheap correspondence course in creative writing: a set number of exercises that you worked on in your own time and then sent them off to get evaluated by a published author. The feedback was pretty vital - it really helps, but the focusing of your efforts in order to produce some work was probably the thing that really gave me my breakthrough. I realised that my problem was that I had never got to the end of writing something! (Also the fact that the correspondence course had an upper limit on the number of words you could submit was a blessing in disguise - quite a few of the assignments were write a short story in up to 1500 words. It's a challenge but I think it teaches you a lot about a whole host of writing issues.) When I had understood that and more importantly actually written a fistful of completed short stories my confidence was much higher and I found myself on the path of aspiring writer!
--> My advice here is to try as many different approaches as possible (and stick with them for a good bit to try them out) till you find the one that works for you.
Ok, ok there's a lot up there that had to come together at the right time so there was an element of luck (perhaps explaining why it took me decades!) and I can't give you a structured-this-is-the-way-to-do-it, because well you'll be different and need different techniques and insights to get you started no doubt, so I'm afraid I can only give generalisations and my own experiences.
Hope you get something of value from all that ;-)