Fill not Working as Expected

Hi,

i need to close a open mesh of a horse head that i downloaded online. Basically i go to Edit Mode select all the lower edges and press F to fill the hole but it also fills parts that should remain open:

It’s a very complex and dense mesh…i know. I don’t know any re-topology or whatever it’s called method to simplify it, is there a nice and easy solution for fixing this???

The link down here let’s you download the .blend file, it’s really heavy: 30MB
https://drive.google.com/file/d/1nDDZjvs0c5xl3Ez8pNrUhdclTchKejq0/view?usp=share_link
Thanks

https://www.dropbox.com/s/uszy0ndlux77quf/Mirrored.blend?dl=0
https://www.dropbox.com/s/6sl61o6ky7ju4jd/Bool.blend?dl=0

Mirrored Version has a double sided horse.
Bool version is like your one but with the problem fixed.

First time using DP so hope the links work.