Hi Sadpapercourtesan,
Been there, done that, lost count of the number of rejections I've had. All very demoralising!
Because I already completed my first book (fiction), in the end I just decided to publish independently - so that's an option for you. Yes it would be lovely to be traditionally published, which will get your book out there in a big way, give you an advance, royalties, advertising. But of course publishing houses have to be very selective in the books they invest in - and oftentimes it's a case of who you know, and how famous you are...
But publishing independently gives you much more freedom to make your own choices, though be prepared to roll up your sleeves and do a lot of hard work with getting a formatter, proofreader, editor, ARC readers for feedback, book cover designer, etc. You also have to take care of advertising and marketing. A bit like having your own little company, but for your books.
Up to you.
MOST important of course is how good your story is (what are you writing by the way - fiction, non-fiction, picture book?). For fiction, it's actually quite difficult to hold readers' interest for an entire book, and keep them invested in the characters, the plotlines, etc - so that they really enjoy reading your work, and want to read more. So get lots of feedback, either from friends and family, or a writing group, or join a writing class.
Keep us posted on how things go.