What do you do with a book that you can't finish?

It's probably just a matter of needing to come back to it with fresh eyes, but I don't think starting something new is absolutely necessary. Have you been doing anything else in between entire drafts and your writing time itself? Reading much? Sometimes staying too close to your work is like staring at the tv for too long and then your eyes glaze over and you have no clue what you're watching anymore.

You say it's a good story and invested a lot of time in it, so there's something to that. Maybe get a bit more flexible. Up your reading time if you can, rotate between several books throughout the week. Anything in other works can inspire a new idea, a new path to take with your own writing just by getting your thoughts to fire off in other directions. Read through your old stories some more, start to finish (this kind of thing actually helps me which is why I suggest it. It sort of helps my thinking set into a nice rhythm again).

And set yourself a goal for each day or week for how much time you will spend on your novel, starting with a shorter amount of time than what you're working with now, then gradually increase it, because I think you might need to exercise your mind away from the project for a while, get some fresh perspective and start over. You're most likely focusing on one problem right now and you might realize later that something needs to be chopped out entirely instead of being reworked. Or be able to shift things around in a more pleasing order so that the rest of the story flows better. You won't be able to come up with any solutions though if you don't exercise your thoughts a little more. And you can't do that while staying too close to this novel for too long.

Write scenarios you never planned for your characters to get a more rounded view of what you're doing. Which sounds like a terrible suggestion considering your problem, but staring at one aspect of the story doesn't make it any clearer no matter how long you do it. You're stretching your novel's world a bit more by doing this and giving yourself less of a confined space within which to work and think.

To be honest, starting a new project does sound like procrastinating to me rather than an alternative to putting your writing off. Sometimes it works, but sometimes you get sucked into it and wilfully ignore the original problem because you don't want to deal with it. It's going to take commitment to start something else and if you're already committed to finishing this novel, even if you've hit a rut right now, I'd say don't do it.

Hope any of that is useful to you.

To add to what Kolta said, have you given it to others to read? Most of the time, betas can spot inconsistencies and plot holes in your story. Maybe having someone read it will point you in the right direction.

I never finished my first ever writing endeavor. It was high fantasy. I trunked it. It hurt. But now I have a new completed novel with two more being written at the moment.

It's not bad to trunk things. It opens up the door to new ideas.

1. You put your butt in the chair and finish it the wrong way. Then, revise.

2. Have writing friends look at it, if you like that kind of punishment.

3. Send it off to the publisher unfinished (you may need to stage your death for this one {that means fake your death}). Post-humous novels are usually unfinished.

4. Start something new, all the best authors say they are most excited about their 'next' novel.

5. Pretend that you are many times published. If your agent/editor/publisher knew where you lived and knew you had another great story in the making, c'mon imagination here, they would sneak into your home, take your manuscript, assuming it's printed out, and place it on your sunlit lavishly furnished balcony (with a stunning view of the river) with your favorite coffee and biscuits for you to find when you wake up after a restful night's sleep. Since you may not have some of these things go back to #1.

Best of luck

I'll concur what others have said: Finish, Revise, pass it off to someone else, and start working on something new. I've heard that a 6-8 week break from a MS can work wonders.

If you find it hard to complete writing a book, then it may be good for you to put the book aside first. Set the book aside, and focus on writing another book.
Then, when the ideas come back, move on to work with the (set aside) book.

Start something new and hope that one pans out. Honestly, I have a ton of unfinished books and novels (one still incomplete at 100K!) I usually just move on.

It sounds like you enjoy short stories and picture books more than long form content. Maybe go back to your first love for awhile.