hi welcome to sr. my heart is breaking for you, you sound like such a loving person. i know you do love your gf but you deserve the same love back and like she said, she can't be that. i'm so sorry.
for someone who don't know much about addiction, you seemed to have figured out most of the answers to your own questions. yes, addiction is like that. my husband of 23yrs is the addict in my life, 2yrs ago i had to separate myself from him and his addiction. i was going insane trying to help him.
what helped me and might help you, remove anything that reminds you of her, the sweater, the music, put it away. make a decision not to allow yourself to think about her good or bad. do thinks that will keep your mind occupied. try not to think "forevere" do what you need to do one day at a time. imo, she has done you a huge favor, living with addiction is so much worse than what it is for you now.
search your area for alanon or naranon f2f meetings, you will find others will can relate to what you are going through and can help you get through all of this and help you to learn to keep the focus on you and not obsess about her.
i know its hard and it hurts but one day at a time, it does get easier. i'll add you and her to my prayer list.