Yes it is mind altering, not like say - LSD or something, but it alters your mind, you think differently, act differently things seem different, it's a drug, it's meant to make you feel different. If it didn't change how you felt, mentally, why would anyone use it?
I guess there is a chance that he and his dealer have hooked up to get fit at the gym and get clean, but I wouldn't bet any money on it. Drugs are hard to quit.
Quitting is just the first step, then there needs to be recovery, from the selfishness, the mindset that is picked up when you are an addict. Drugs change the brain, he is not recovered (or he's a walking miracle).
As Aries said, it is so important for you to have support right now, Al-Anon, that book I recommended, therapy.
As for your question about people bailing on relationships after rehab, yes, I've heard that a few times. Could be that they want to start fresh, could be that they want to leave all that hurt and pain they caused behind, could be that they aren't thinking straight, well that's kind of a given, the not thinking straight.
Regardless of his reasoning, he's doing as he pleases and you should too. You deserve to have a relationship where someone is kind to you and treats you with respect, that's not this guy, right?
As Aries said, you will not always feel this way, you will get past this but look for support, here at SR and with say Al-Anon, start a journal, be real in that journal.
He has hurt you badly with his erratic behaviour, you deserve so much better.