Sarah,
You're not stupid, so let's get that straight.
You've been in love with an addict and their addiction ends up taking first place in their life; not you. That produces confusion and questioning everything you do with regards to him. One minute they are like, "I love you and appreciate you so much" to ... nothing and you feel ignored, which you ARE ignored. This is the dance of dealing with addicts. It all depends on if they are high and feeling good to low and needing the next hit or fix. So when people say it's a rollercoaster that's a good way to describe it. Or, a yoyo....up and down. Up and down.
Sure, he's going to feel more "comfortable" with another user or someone who has no issues with his drug use. That's the "easy" direction...he doesn't have to face his issues around her; his conscience isn't uncomfortable. This in no way detracts from YOUR worth. This is really about his addiction, not you....I hope you come to believe that if you don't already.
I wouldn't say you're "hanging on" as much as you are just trying to sort out the feelings you still have. The breakup is still fresh and it takes time to get over it. I think it's natural to question yourself.
At this time I hope you put yourself FIRST right now, focus on yourself and be extra extra good to YOU...and try to maintain no contact. Because from what I've gathered every time he contacts you it stirs up things that are maybe left alone.