ITunes waiting for changes to apply?

When I try to sync my iphone to my itunes on my Mac, I get " Waiting for changes to apply. If I wait long enough (like and hour) sometimes it will start to sync, but it has taken all 2000 songs off my phone and is now reloading them. (l have just left my phone overnight to get it to do this) Now this last time I get up in the morning and sync canceled half way though. I have more than enough room on my phone for this, so it's not a space issue. Went to apple sight and here so far and not finding allot of answers, just a lot of people with the same issue. Hoping someone has found the fix.
Thanks

Added (1). PS I have updated both the phone and the mac

"sync my iphone to my itunes on my Mac, I get " Waiting for changes to apply. If I wait long enough (like and hour) sometimes it will start to sync, but it has taken all 2000 songs off my phone and is now reloading them."

With iTunes open, click the top menu bar at "iTunes" and choose "Preferences".
Select "Devices". Do you see a checkmark for "Prevent iPods, iPhones, and iPads from syncing automatically"?
If not, the sync process will not only erase all the music on the iPhone to add every song that is on the computer, but also do the same with…
ALL Albums, songs, playlists, movies, TV shows, books, podcasts, and audiobooks.
ALL Photos and videos.
ALL Contacts and calendars.
ALL Device backups that you made using iTunes.

That's a ton of stuff. It takes a long time just for the computer to erase the iPhone and organize all the data that is to be copied. It's an amazing process that we take for granted, like seeing a dog walk on its hind legs.

With a checkmark at "Prevent iPods, iPhones, and iPads from syncing automatically", you can choose to sync only one or two categories. Before you experiment with manual, selective syncing, be sure to make a backup of the iPhone. After all, it probably has 300 valuable selfies on it.