Cannot transfer or access music from imac to iphone 4s (sync fail)

Hello, I am using an iphone 4s, iOS 8.4.1, manually managing music, capacity 56.6GB. Last week, I tried to transfer some new songs to the phone from my iMac (OS 10.10.4 Yosemite, 2.8GHz Intel i7, 16GB RAM) via itunes and a direct cable-tethered connection. The first set of songs that I transferred went over fine (~20?), but then I tried to add another 15 songs and take off a few and that failed. The songs were showing up in tunes as gray with dotted circles on the left side. I've had this before and said "ugh."

I noticed that my storage space was low, so I tried removing all of the photos and then re-syncing. No luck. Several times I have tried restoring from an old backup from before the music transfer, and that also didn't work. Worse, now my phone seems to be unable to find songs that were previously available and when I click on the song in the music app the app gets unstable, sometimes crashing, but mostly the song turns light grey and there is a small icon of a phone in the upper right quadrant of the song entry. Now I cannot access *any* of my songs *except* for songs purchased from the itunes store. I have seen this on the forums, so before one of my restores, I tried rebooting the imac with wifi turned off, but that didn't make a difference. Also, I have tried to set my music to sync to a playlist (with my previous songs) instead of manually managing, but the songs did not transfer after the sync.

I have had problems in the past with music transfers to this phone, as well as to an iphone 5 and 5s in the family, and I am close to giving up on Apple products altogether. However, I am hoping that someone in the forums can talk me down from the ledge and help me fix this issue. Thank you in advance!