Wow this must be awful for you. I'm so sorry. It's so hard for a woman to move on with no "closure". It's upsetting that he didn't give you a reason for leaving. I also get the feeling that he may have started reusing and didn't want to dissapoint you.
But on the flip side, maybe he felt he couldn't express himself after he got off the pills. I know my XABF could only be emotional and expressive of his feelings while drinking. When I look back, every moment of "bliss" and happiness were while he was drinking. He even told me he loved me for the first time while drunk. When he would go through his dry periods due to my nagging, he would be withdrawn, crabby and unemotional. He obviously felt the need to drink so he could express himself. So maybe your X felt so happy and in love with you because he was high. Then, when he stopped, he couldn't feel the same way anymore. I'm not sure if I expressed that correctly. When I am drinking, I feel way more lovey dovey with whomever I'm with. Since I'm not an addict, I know what's real and what's not while sober. But to the addict, they only know how they feel when their high or drunk. They lose the ability to discern real feelings because the drug of choice motivates their every emotion.
Now thats just based on my own experience. Every experience is different and I'm still learning. Just thought I'd throw my 2 cents in. I hope you begin to move forward and realize that there are so many other people out there who deserve your love. My Mom said to me today, I'll bet you wouldn't be so depressed if you had a boyfriend. This sounds silly, but she told me that she knows I'm not ready to move on yet, but eventually that day will come. And when I'm ready to find someone, I won't miss him as much because I don't really miss HIM. I miss the IDEA of HIM. I agreed and it helped me feel much better for at least the moment.