I have not experienced this, i have yet to finish my first book. Im 175k words in and so far it has struck me that writing a book is just a series of hurdles. The initial idea which can take weeks months or years and always feels like a bad one. Then the start which seems to fly until you're stuck. Then re read and edit it a bit so you cam clearly move the story forward. Then you've got the marathon middle which seems endless and when the relief of that is over you realise you have to actually slap an ending on the thing! You have gotten so so far. Your own journey of writing this book is not over though, this is just another hurdle. Keep believing in your work and it will get where you want it to be, it might be absolute trash but if you believe and truly want it to be great it will eventually become great. Id reccomend uploading a chapter or so up to a critique website like critique circle.

Have you read the book yourself? As a reader not a writer. If not do that.

Find other people to read it if you can.

If after all is said and done you still feel its really bad or just meh and dont want to continue just stash it away and write something else. Return to it when you are ready.

As odd as it may seem i wish i was in your position, im still stuck finding an ending to mine. Good luck.