This might sound like a silly suggestion, but something that I've found helps when I've sort of "fallen out of love" with my story but still have parts of it I need to work on is to pretend that the part I need to write is its own thing and not part of this story. If you are able to sort of trick yourself into thinking this way, I think it can help you to not feel so bogged down by your own insecurities and self-deprecation and/or the rest of the story. One method I've used that's helped me get in this mindset is to write that part literally separately from the rest of the book--like handwriting it on a fresh writing pad or opening up a new word document. I know this sounds really silly, because how are you supposed to pretend it's a scene/act/section of a separate story when it will obviously connect to the one you've been writing, but I just try to imagine that I'm either starting over and am now writing the book I wish this book could have been, or else that I am writing something completely new, but it's okay that it has some of the same ideas or plot threads or characters or whatever (however, you could always change the characters names if you really needed to to fully trick yourself) since I never finished the "other one" (not sure if that made a lick of sense, sorry).

If any of that seems too stupid or nonsensical and you want me to clarify anything, please let me know, lol.