Publish or Paralysis:
I am finding I have a similar problem. In my case, the second book is intended to be in the same series. Only how do I muster the will to write a sequel when I haven't gotten the first approved?
Getting that first book written is a Herculean task. You are completely devoting yourself to achieving a grand task you always doubted you could EVER finish, let alone start. BUT it was a consuming passion, and you felt like you couldn't face yourself without achieving this goal.
Guess what? You did it! Kudos! But now you did the big, impossible task. Now you have to face a bigger, more daunting and less certain task - to BE a writer, possibly to be a life-long successful writer of numerous books. The goals are more vast and overwhelming than you can imagine.
The same secrets that get you moving still apply, though. Break your tasks down. First, there are tons of steps before you have the first one publishable. Have you done all the editing, finding beta readers, RE-editing, finding editors that fit your needs, RE-re-editing, and either seeking Literary agents OR self-publishing houses? All this stuff is needed to have a book that can really be published, and if you haven't done it, you aren't finished.
If you promised yourself something for writing the first book, did you do it?
Now pick a new book to write. I recommend outlining for this, since you have likely spent YEARS rolling around the ideas for that first book. Outlining can get your blood pumping. Then research. Does the Hindu god Kali feature in your story? Do you need to have an idea of how to describe playing the guitar? Research helps make starting writing feel less overwhelming as you smooth some of the hills before you.
Then set realistic goals for continuing. spending a small set amount of time writing each day means you at least do something, even if it's just 15 min. Once you get passionate about it, you are likely to blow past these goals. Remember they are minimums. You may need to do more enjoyable tasks other than writing to keep yourself happy. This is why you don't want to make these goals TOO high. We are still living in the land of COVID (I got my second vaccine dose yesterday and feel kind of run down).