Writing the second book in your series will often times seem a lot harder than the first. If you've read "How I write a book", perhaps you will have a hint as to why.
Is it because it's difficult to start? Nah. Is it because you used up all your good ideas in the last book? Nope. I'd say the big reason is that you'll end up comparing book 2 with book 1.
"But, but I'm a better author than I was when I wrote the last book! Shouldn't book 2 be better than book 1?"
But if you recall, your first draft of book 1 was barely legible, yes?
It's like when you go out to dinner. You're hungry, you order the food, you eat the food - now you're full. Then the bill comes. That seems like an awful lot of money, considering how full you are. You're looking at it through a different lens.
When you look at your shiny, beautiful book 1, all pristine and edited a kajillion times, with its fancy cover... and then you glance at your borderline illegible book 2... well, it can be tricky to remember the process book 1 went through to look that way.
No wait! It's like going and getting a professional makeup and photo shoot done, then looking at yourself in the morning the next day in the mirror.
Yes. It's like that.
Anyway, analogies aside, you have to remember how hard you worked to make book 1 into the masterpiece it is, and simply trust in yourself that you'll be able to do the same with the steaming pile of rough draft you have in front of you.
As always, my advice is the same: Write a terrible book, edit it into a decent book, beta read it into a good book, edit it again into a great book, and maybe, just maybe, find someone special in your life who can make it phenomenal.
(even if they do kill far too many of your commas)