I would suggest you plot and outline your stories more before writing them. Also, how long does it take you to write your 2 -1112 pages? If you take too long to write a story (more than 2-6 months) you'll often run out of motivation. Writing a book (specifically fiction) should not be a leisurely activity. You need a game plan (your plot//outline) and at the very least a rough schedule for writing. It could be 30 mins a day, 3500 words a week, etc. But you need something to aim for, to keep you on track, to keep your head in the story. If you write once every two weeks, obviously you're going to lose steam and get frustrated. You need to have your all in your book, continually, until it's finished.
I will say, it takes a while to learn your own creative writing style//pace. I had 4-6 different stories during my early writing years and they were all terrible and I had no idea what I was doing. But I learned a lot. Slowly I came to understand my writing process and the steps I need to go through. And a very VERY important step is the plotting//planning process. It's super fun to get an idea and start writing it immediately, but sparks do not make stories. You need your spark and your ideas and mix it together with time and brewing. Let your stories simmer in your mind and take shape. This can take a few weeks or a few years. The book I'm working on now I've written from scratch 3 times, starting in 2014, and I'm only just to the point now that I have a mostly full plot in place and I'm rewriting//revising it. I consider this book to only just be past it's infancy and I've been working on it for 5 years.
Read books about how to write. Find blogs of authors and fellow young writers who are struggling along with you. Study the craft. Try new things. Figure out what doesn't work for you and what does work for you. Learn what your creative process looks like and learn how to make it work better. For me, all of my stories go through at least 3 major evolutions where my plot and characters and settings and everything can be completely rehauled and changed. Three times at least. I used to get frustrated, but now I know to let my stories simmer and take their time, plot and play with them, and once they hit that 3rd stage, I know I'm ready to start writing my first draft. I can appreciate the time and effort and love my stories go through, because I understand my process.
Also, go easy on yourself. Being a young writer just entering the world of writing is really tough. There's a lot to learn, a lot to do, and no clear roadmap because everyone does it differently. Enjoy the process. Be patient with yourself. Learn everything you can.
I would specifically encourage you to learn how to plot your stories better, in response to your question. I found this amazing article yesterday that may help you learn how to structure your plot.
https://www.advancedfictionwriting.com/articles/snowflake-method
I also encourage you to check out NaNoWriMo. What you write for NaNo may be trash (writing 50,000 words in 30 days) but it will teach you how to be dedicated to writing every day, take your writing seriously, and to create a full story in a short amount of time.
http://nanowrimo.org/
Suggested blogs(will add more links as I think of them):
https://www.abbiee.com/