Don’t know what to do

So basically I have known my current boyfriend since we were young. I have always been like obsessed with the kid. We had a little thing back in the elementary school days. He’s a year older than me, so currently I’m 19 and he’s 20. We both went through some shit in life around middle school from both of us having drug addict parents, to each of us losing a parent as well. I took that stress and grew with it and he ended up going down a dark path, so I only heard from him on occasion through the next couple of years. But I have always been super close with his whole family because I’m also his cousins best friend. It wasn’t until December of 2017 I started to talk to him again, he got off the drugs he was using(meth, coke, Xanax), wasn’t drinking, so was sober for a whole year. We started talking again. Then in March after Talking for a few months we start dating since literally we were amazing together and were like bestfriends.

Then now here I am 6 months later. We haven’t had any major issues and the issues we have had we settled like mature adults. This has probably been the best relationship I have ever had. The Last 2 months he began hanging out with some friends he shouldn’t of been and his drinking was getting out of hand. He recently began to relapse again on meth. He stopped his support group classes. We used to have the best communication and now he just yells at me. I have PTSD from my past with the abuse I went through growing up as a kid from my father and more. My bf knows it, but when he gets drunk he gets mean and acts like he doesn’t give two fucks about me all of a sudden. It has started to worry me horribly. I knew coming into this relationship there would be struggles and relapse may happen, but it’s just getting out of hand at this point. He had nothing before basically we started dating, now he has everything from a truck, to a $17/hr framing job, we just signed a 12 month lease to our own apartment, and we took on his little brother because his mom is dealing with her own addiction. I am in school for nursing. Like everything is great with us. So I don’t understand why he has decided after over a year of doing good to start going back down again and idk how to help him cause I have tried a lot already. It’s like Hills with the last 2 months. He does great, then goes back down, does great again and as u guessed goes back down, it’s been a cycle. And On top of this all I’m 4 days late on my period. I think it may just be stress, but I’m not completely sure. I mean I took a pregnancy test the first day I didn’t get my period and it was negative. So I don’t think I’m pregnant. I just don’t know how to have him help himself get off this path he got on again. And I can’t just grab my shit and leave him, id have no place to go and I very much love and care for him. So if anyone has any advice what I should do. And if they think me missing

My period is due to stress or could I be pregnant and the test was just wrong by chance?