The first two hurdles are: one, coming up with an idea; two, starting the first draft.
Number three is finishing the first draft. I don't know how many times I've met someone who's started a book but never finished.
You need to finish. Finishing your book is a huge accomplishment.
Start by setting a daily or weekly word count goal. It can even be 100-200 words a day (which isn't much, but over time the words will add up). Then, after a few days or weeks, slowly increase the word count. It's okay at first to not write everyday. Go every other day, if you want. Or just on weekdays--or weekends. Although I do not suggest the latter because there's too much time between the weekends. You really need to be working during the week too.
Ideally, write every single day. Even if it's a small word count.
Take a 75,000 word novel. Break it down into 500-word days. That's 150 days. 5 months. If you were to increase it to 1K or even 2K a day, you see where the number of days is shorter (a 75K word novel, in 2K a day chunks, can be completed in a little over 37 days!)
Set a goal.
Then achieve that goal.
Then, strive to exceed the goal. You can do it.