How can I tell what's going on with my girlfriend?

My girlfriend and me have had a hard past year. I’ve made some major mistakes, used drugs, not exactly been the best boyfriend for her, by any means. A few months ago I made a major turn around have been doing a lot better since. At the time, I apologized to her and we made up and for a while our relationship seemed to be getting a lot better. But in the last few weeks she hasn’t been talking to me as much, she takes a long time to write me back when I send her a message, and even when I do see her, I just get a vibe that something has changed. I’ve asked her if things are okay, and if she still loves me and wants to do this and she said yes, and has confirmed it several times when I’ve asked, but she’s never come out and said it on her own. From the outside it would look like everything is fine, but I have a really strong feeling that something is wrong, it’s making me physically sick, it scares me so much. I’m desperately in love with her, she’s the most incredible person I know, and even though I’ve done more than enough to drive her away, it’s killing me to worry that she’s drifting away from me now that things are finally going so well for us. How can I find out what she’s really feeling and thinking right now? And if she is starting to lose interest, is there anything I can do to bring her back to me? How can I make her more interested in our relationship again? Is it possible that I’m just being paranoid?