Trying to understand meth addict.

• Call me codependent if need be, but I could use some support.
  
  I got into a relationship with my ex unknowing at the time that he was a coke addict.. he eventually progressed to meth as things got worse and eventually I found out and I broke it off with him and moved out. Things got so bad that he got arrested for the first time, then he eventually overdosed. One of his drug friends actually saved his life and got him to the hospital. Eventually another friend of his figured out what was going on and staged an intervention. He got him into an amazing rehabilitation center for 2 months. It seemed to change his life. Right after detox he started writing to me a journal or sorts. Realizing and remembering all of the things he did to me. Making amends, apologizing, saying how much he missed and loved me. It ended up being 30-40 pages long. He decided to go to sober living in another city after rehab. The minute he got a cellphone he called me and decided to help me through all the pain he caused me, by calling every night - talking through all the painful memories. And let me tell you, at times he took a beating. It wasn’t easy and I told him it wouldn’t be. I also expressed this may be too soon for him and his sobriety to take all of this on. But he said he accepted the challenge. Whatever it took to help me. He eventually found an amazing job after being there a couple of months. There was some training that needed to be done back home, which I knew would have been a bad idea. We discussed this in length and he said he already has meetings mapped out and that he was going to be fine.
  Well, since he’s come back home, he’s started acting suspicious. Giving excuses, rarely communicating, not going to meetings, even having panic attacks when talking to me. He turns into a completely different person when using. So, I decided to break it off with him as I know what’s going on.
  I can’t understand how someone who’s come so far, who got this amazing second chance at life could just decide to just mess it up so easily. He’s definitely already reached his rock bottom.. I guess he has no rock bottom.
  I’ve decided to start counseling, but I still have so many questions. As much as he’s tried to help me understand addiction, I just don’t. All of this pain has come back and I just feel stupid for letting him do this to me again. Unknowingly while we were together, I had only gotten a small glimpse of him sober, and I wanted to see what he was like now that he was “finally done with drugs”. I just didn’t see this coming as I saw and felt a real change in him. But now I’m just left with so much hurt and worry.