How can I keep at my writing without stopping?

Hi! I have this problem all the time, and it really infuriates me that I always get bored of it. Here are a few tips that I've learnt about how to stick to one story as I've gone alone:
1.) Don't plan it too much! Yes, you must plan it, but don't make it so that every single detail is planned, because then it's far too predictable for you, and your mind will drift to other ideas that will lead you to another story. I have done far too many very detailed plans, and at first, I weaved new ideas into it, but the whole idea of it just got repetitive and boring, and I began a new one. So plan it, but not too much.
2.) Don't weave ideas into it! Sometimes ideas are great to tie in, and you get distracted by something else. Ideas from it will come into your story, and that's OK, unless it gets over the top. I once wrote a novel that was the longest I have ever written, but it just got more gruesome, more horrifying, and as far from fantasy/mystery ever.
3.) Other films coming out? Good books that you're dying to read? Wait for it. You can't read or watch anything that's far too different from your story, because it will end up changing everything about it. If it's the same genre, then fine, go ahead, but try to avoid anything else. It WILL ruin it all. It will only distract you.
4.) Listen to other people's advice. These answers are all good in one way or another. Don't just listen to one person. Listen to everyone, and take criticism. If you don't, then your story may just be a load of rubbish, and all because you didn't heed people's words. Of course, if they say "change your whole plot", don't. :-)
That's it! Good luck, and I hope I helped even a little bit.

You just have to do it. Give your whole attention to one novel and stick to it. I had the same problem at first, but I got a notepad to write down all my new ideas so that I didn't scrap the one I was currently writing. Also, it helps if you really love the story of the book you're writing :)

The most important thing is finishing what you start. If you write for a month and abandon something, you've wasted a month.

The great thing is that while you're working on one thing, all those other ideas are still working on themselves in your subconscious. They're developing, incubating. Remind yourself that you're not JUST working on what you're currently writing. Your brain is doing double-duty, and when you finally do get clear of your current project the ideas you have waiting for you will be stronger and more fully-developed than if you tried to switch to them now.

Let them simmer and they'll boil over onto the page when you're ready for them.

Curious, I am just like you. I have started many novels and only really gotten 3 or 4 chapters in before the going-ons in life pulled me away long enough for me to lose interest. And like you I have many ideas for mine that I know full well can NOT all go into the same novel.

Currently I am writing one and the only thing I have done different than before is NOT OUTLINING. Yes, I came up with a plot line and a general series of events and much of that has not changed but a few key portions such as character relationships and how the same events come to pass. Such as my MC's mentor was originally going to be the good guy throughout. After 3 chapters in I began thinking of making him more of a manipulator of the MC, who won't realize it until too late.

Also, my novel ideas are all fantasy and science fiction. To me fantasy = magic has precedence over technology, sci-fi = technology has precedence over everything, magic non-existent. And my beliefs are druidic so naturally my MC is a druid, and one thing that has helped me retain interest is investigating other sources for ideas of abilities to give him. Such as WoW, the Shannara series, DnD, etc.

Outlining has always been my trap. If you do write one, DO NOT specify as much as possible. Events and characters only really, as you may feel when you get further along that those same events may come to pass in an entirely different manner than you originally foresaw. And when you do have one of those thoughts, that alone is inspiration enough to press onward.

Write down the ideas and then continue writing the novel.

Try this method of writing, free writing.
1.) Write all inspiration in your brain, don't stop just because of wrong typing, punctuation, capital letter and etc
2.) After you finished, leave your writing for 1 - 2 days
3.) Open it and fix the wrong typing, punctuation, and etc
4.) Done

like the others saying, write out all your "else" ideas and out aside, focus on the piece you are having now.

it can get really hard sometimes, but try writing down all your other ideas, and just stay foucas on the one your writing. Usely doesnt happen to me because once you get focased i cant think of anything else