There are many challenges to writing (as well as delightful moments), but perhaps the most difficult is the will to persevere, especially with the first book.
Self-doubt racks many writers. Writer’s block can be a problem for some as well.
Remember that the first draft isn’t meant to be a polished shiny diamond you can present to the world. It’s a roughly hewn lump of rock. The first draft is just about hacking the damned thing out of the earth (a good description I read somewhere or other was that the first draft is when an author tells him/herself their story).
Just keep going. If you can’t write a lot, write a little each day. You can improve the writing quality, expand upon some sections and cut others during redrafting.
About halfway into the first draft might be the most difficult time, especially for a first story. Initial enthusiasm has been whittled away, and the light at the end of the tunnel is barely visible. If you’ve written something before, just remember you probably felt this way then, and you got it done. And if you did it before, you can do it again. If it’s your first, remember that most writers go through an awkward phase when they’re far from the beginning and far from the end.
Redrafting might seem lovely or might seem horrid, but it’s vital to spend the necessary time cutting the chaff and allowing the wheat to shine forth. It’s usually a good idea to get some objective people to act as beta readers, and to take account of their views (if a beta reader points out a flaw you can correct it. If a reviewer points out a flaw it’s there forever, and even if you amend an e-book the review remains up).
Writing novels is a marathon, not a sprint. Just keep on going. Don’t be afraid of walking very slowly when times are tough. Slow progress is infinitely better than no progress.
Perseverance is the single most important quality for a writer. More than marketing ability, more than writing ability, more than being best friends with editors of major magazines/papers. If you give up, nobody will ever read your book. So, keep buggering on.