I can't add/delete songs from my phone on iTunes

I have a Macbook Air and an iPhone 5s. When I plug my iPhone into my computer, there is no "add to" option to add songs to my phone. Also, when I click the songs on my phone, it gives me many options like "Get info" but no "delete" option. It is incredibly frustrating because I would just like to add some songs from my computer to my phone. I should also note that the drag & drop option is not working either. I used to buy songs on another account and I have songs on my phone from other libraries that I don't want to lose, so I don't want to delete them all & sync the whole library that is on my Mac. If anyone knows what the problem is, I would really appreciate your help. Thank you!