I can’t stick to the boundaries I set

I am new here - my boyfriend (we are both in our early 30s) has had drug and alcohol problems since he was 16. We were friends in high school and then rekindled a little over a year ago and then started dating. When I started dating him he said he had gotten out of rehab over the summer and was not doing drugs (coke is his drug of choice) and only drinking a couple of beers on weekends. However, for him, alcohol always leads to weed, then he thinks he can handle that, and then he starts doing coke again. This is what happened and I found out he had actually been using coke for our whole relationship (for about 7 months at the time) and it got to the point where he couldn't hide it anymore because he was doing it so often and his behavior was so off that he had to admit it. He got sober (actually sober with no drinking either) starting in June and I had a lot of fear and anxiety around staying with him after all of this because obviously my trust was completely broken in him, but he convinced me he would never do a drug again and would not drink and that he could get sober and fix our relationship. And he did. Our relationship was great from this past June until October. It was like I was dating a whole different person. Then in October he drank and now you know the rest of the story because history repeats itself. I had told him when I stayed with him the first time that I couldn't be with him if he used under any circumstance. Now I feel like an idiot because I'm still here and he's smoking, drinking, and doing coke. I've never dated an addict before this. He is a master at lying and hiding things, but I knew something was off when he started to avoid me and knew it must've gotten to drugs. I've tried to help him, I've tried talking to him - nothing works. I've talked to him about it (has only agreed to see me a few times in the last two months) and I see glimpses of how he was when sober and I know he cares about me and loves me, he is just not ready to stop. He also has blamed things on me which is really upsetting because everything that he is mad at me for (getting his family involved because I can't be the only one knowing this information, showing up to check on him because I hadn't heard from him and was afraid, etc.) is because of his choice to use. I know I'm an idiot. I feel stupid because last time he was begging to stay together when I found out, and this time it's like he doesn't want me but he keeps me attached because he will say he'll stop and not and things will get better for a few days then go back. This has been really bad for almost two months now - that's when it got to coke. I'm so frustrated. I am scared to leave him because I don't want anything to happen to him. I told him if he doesn't talk to me/try/get help I can't be with him and yet I'm still here. I can't get myself to leave. We are "together" still but I feel so alone. I cannot stick to my boundaries and he doesn't take me seriously because of it. Will this cycle just keep happening for the rest of our lives if I stay? I know this was mainly a rant/vent but any advice is appreciated - I am really lost.