In case you thought you're alone struggling with the whole writing process getting easier. I've got news for you. It doesn't.
Every book is different. And every year your life is different. Hell, every month even. Every week. Every day. You forget things. Your attention shifts to something else. Your goals get a makeover.
However, there is hope for you (just as there is hope for all of us).
By consistently practicing your craft you get better at it.
It's just as hard to write your 7th books as it is your 1st (yes, speaking from experience). The difference is, you know exactly how hard it will be, you get ready for it, and then you do it. The fact that it's hard no longer stops you.
So keep writing—keep moving forward. Keep practicing as often as you can.