Maybe you can use some tape to keep parts in place.
I know that tape won't fix permanently (or at least isn't meant as only thing to keep 2 things together), but if glue isn't working it might help just that bit.
Working with fibreglass really isn't easy, I found that out the hard way myself too.
Fortunately I was doing a study with lessons on how to work with polyester and fibreglass and more resins, so it didn't take long before I learned how it should be done.
As I wasn't patient enough at that time, I already had ruined a hull, which even today is as smooth as the Himalaya
Applying the fibreglass properly is really important for the finish.
Apply it the wrong way and you'll end up with the Himalaya, but when applied the right way you'll hardly need to do any filling or sanding.
Greetings Josse