There comes a point in almost any extended piece of writing when you run out of steam. The initial burst of enthusiasm evaporates and you find yourself shrinking from the half finished story like a friend you have fallen out with whom you now wish to avoid at any cost.
As someone who has worked as a manuscript doctor as well as a regular writer, I've seen enough of these promising but uncompleted works of fiction to fill a builder's skip. The writer always want to know whether it's worth carrying on and my advice is invariably the same. Finish the bloody thing even if it kills you. It might actually turn out to be as good as you imagined in your first shining vision. Alternatively, it might turn out to be dire. Whatever the outcome, you will learn a lot.
The best things we do are very often those we are forced to continue with even when we are sick to death of them. You can't give up on your children even though it's four in the morning and they are refusing to sleep. You are obliged to soldier on because there is no alternative. In the process you grow into so much more than the person who once fondly imagined parenthood as a wonderful and entirely painless adventure.
The same rule applies to writing. You have to persevere and find a way through the barrier of creative fatigue. If you do so, at the end you may have produced something wonderful. At worst you will have something to measure yourself against. Whereas if you simply throw in the towel, stick your half completed manuscript in a drawer and forget all about it, all you will have gained is a sense of failure.