using purchased track as ringtone

this is utterly outrageous how much difficulty i have faced in trying to make a simple song my ringtone. i have purchased the song from itunes and it won't show up. i have changed it to an m4r track in itunes. i have tried to upload it into garbage band to make it a shorter track so it would be the length of a tone. what on earth is the issue? why can't i use the song i paid for as my ringtone? i am baffled. i've spent days researching trying to make this happen. apple, what gives???