Import not working, iTunes buggy, I need help with this

So, the problem started a while back. Me, being a music lover, was heartbroken to find out that the iTunes store did not sell many of the songs I enjoy from various artists. Being the intelligent person I am, I decided to import music I had purchased elsewhere on the internet to my iTunes, which I would then import into my iPhone 5. The first day, everything worked great. I imported a few songs and was satisfied. The next day, however, when I went to import more, a bunch of huge issues appeared: Namely, incorrect data usage being shown, crashes, and more. In order to import, you select the iPhone (Or device) logo, go to music, and click 'Edit playlist' to change the music on your phone. After that, you simply drag and drop from your iTunes library to your device's library, and voila, new music. However, this time, it didn't work. Even after dragging and dropping, both albums and individual songs, nothing would come on or off my phone. Later, I also realized many, many songs I didn't want nor have were appearing on my iTunes' playlist. iTunes said I had 259 songs on my phone, but when I clicked 'Edit playlist' it then showed I had 236 songs installed. On my phone, however, it says I only have 257 songs installed. And that's just the tip of the iceberg. Sometimes my iTunes will crash randomly, as well. I haven't tried re-downloading iTunes, but it's been updated several times recently and that should have fixed any problems, but no.

Sorry if this was a confusing read, but I'm desperate to get those few songs onto my phone and clear up anything that's wrong. I've looked other places, but nothing helped. The main things I need to fix, in order of importance to me, are as follows:

Importing the music I own (But not on iTunes)

Fixing the incorrect data usage shown in iTunes

Fixing iTunes and its crashing

Thanks is advance for the help!