Having trouble syncing my music to my iphone?

I'm having trouble with syncing mucic from itunes to my iphone 5.
Basically, I also have an iPad with more memory. I want all my music on my laptop (in itunes), all songs that are checked within itunes (not on the iPad settings) on my iPad, and only selected playlists on my phone.
The iPad is working fine. I've checked 'entire music library' in the music tab when my iPad is connected.
My phone has also been working fine until I tried to change the music today. In 'selected artists, playlists… Etc', I had chosen specific albums to sync, and it was fine. Today, I made a playlist of about 100 songs in iTunes, and I went to sync JUST that playlist to my phone. But, when I unchecked everything else in the 'music' tab of my phone settings in iTunes, and checked that playlist, then clicked 'apply', it all reset. As it was syncing, all my playlists that I'd already created on my phone were re-checked automatically, and my new one was unchecked. I went back on my phone and nothing had changed.
To test, I tried completely unchecking 'sync music' at the top, and the exact same happened again when I clicked 'apply'. It was re-checked, and so was my original music.
This is really annoying, as I have little space on my phone and I really want to have just that playlist of 100 songs on my phone for tomorrow.
Any ideas why this is happening? Am I missing something?

I don't understand why you don't just use manual management and then drag the songs to the iPhone tab on iTunes. Honestly that's the way I do it and I never have that problem.