HTC One M7 :: Music Slowness And USB Syncing

I've had my HTC One about a year now after having iPhones for several years. I also have a Mac. There's a few things that I haven't really understood about the phone and I'm hoping to finally 'get it' now. Here's the issues I'm having:

1. I am using Google Music Play to listen to music and see everything if I select "All Music" from my phone. But last month I listened to too much music (apparently while not connected to wi-fi?), exceeded my data plan and got charged extra. So my question is why is only some of my music actually on my device, and how do I get it all on there?

2. I tried syncing to see if I could get my music to upload and my computer doesn't recognize the phone any more (it used to). I've tried Android File Transfer and HTC Sync Manager (I don't honestly know what the difference is.) Both tell me there's no phone connected. I've tried different ports, but I only have one cable so I can't try a different cord. The phone does charge when I have it hooked up to the computer, so there's SOMETHING working.

3. For the past couple of days, my phone has periods where it's very slow. Text messages take several minutes to go through, the screen and apps seems to hang. I have restarted the phone several times. Sometimes it works, sometimes it doesn't. It's irritating when I'm in the middle of a text conversation with someone and there's a 5 minute lag time between receiving and sending texts.