Storage issues iPhone 6

So apparently my iPhone's memory is full and I cannot acces Whatsapp or others app because of that. The actuale problem is that, after consulting my Storage Usage in Settings, I discover that what I have on my phone only takes around 5.5GB and not 11.6GB . Even after deleting some apps, nothing has changed. To mention that all of my media is upload in iCloud (the original-quality) and I have optimized version in my phone's storage. Not to mention the fact that I want to stop storing them in iCloud and it will take me hours to download a number of 2500 worth of photos and videos in my pc from the iCloud simply because I don't have aroung 2.0GB in my phone to download everytings there and then just simply import everything on my computer.

What can I do in order to fix this problem? Or should I say: problems?

Thank you in advance for the help,

Bianca D.

iPhone6, IOS 9.2.1