Petloss and guilt for being an awful cat mom (21+)

I am completely and utterly consumed with guilt, regret, and self hatred. I know this probably isn't OCD related right now, but I really need to talk to a group of kind, supportive people. I've only ever received compassion here. My kitty passed away 2 1/2 weeks ago. He was with me for 16 1/2 years. I had him since he was only a few days old. He took care of me through every hard thing I've ever been through. My baby, my brother, my teacher, my everything. From ages 9-26. I'm feeling a lot of self hatred. I did so many things wrong throughout his life. But the worst thing I ever did is haunting me and making it difficult to grieve. When I was 19, I started smoking weed daily, multiple times a day. In my apartment. With him in the same room. I would always crack a window, but there was also a 6-or-so month period where I lived in a place where the windows couldn't be opened. And I smoked inside daily anyway. I continued on like this for about 5-6 years. Cracking the window a bit, but not even doing a good job at blowing the smoke out. I never blowed it in his face, I was always concious of trying to keep at least some small amount of distance from him. It probably made 0 difference, I was smoking inside mere feet from him. Smoking weed around pets was very normalized among the people I knew. I knew that it wasn't a good thing to do, but I didn't take it seriously because he seemed happy and healthy. Until about twoish years ago. Coughing fits started. He was xrayed and diagnosed with asthma. I made another stupid choice, and instead of quitting or at least moving my smoking outside, I got air purifiers for my apartment and started smoking in the bathroom exclusively with the door shut and the fan on. The air purifiers were always green (unless I accidentally left the door open which happened on occasion), and I couldn't smell weed in the rest of the apartment. If my girlfriend smoked inside, I would not be able to tell unless I went into the bathroom. There was barely any gap under the door. I thought I did a good job. I thought I was responsible. We also switched his litter to a dust free one. The coughing stopped. I thought this setup was good enough. Something he had done for years prior to this was very deep breathing when in REM sleep with forceful exhales. I always thought this was just a quirk of his because it only happened when he was in a deep sleep. I now know that this was probably a symptom of asthma, and he likely had it long before he had his first coughing fit. My girlfriend and I always thought it was just him having intense dreams. I'm disgusting. After his first coughing fit, he was supposed to get a preventative inhaler twice a day for the rest of his life. But like the fuckup I am, I fell off of doing it after his coughing flare subsided. He had two other instances of coughing in the last two or so years, but only a single cough at a time and not accompanied by any other symptoms he showed during his initial "flare". But he was still doing that deep, forceful breathing when in REM. Sometimes, for brief periods, he would also have a slightly elevated breathing rate (32-38), but I always for some reason chalked this up to post-surgery discomfort (it happened more frequently a few months after he had pacemaker surgery) or one of his other conditions. My dumbass didn't piece together that my continued bathroom smoking could still be irritating him. I still didn't see that sleep breathing as an asthma symptom because it stabilized when he woke up or went into a lighter sleep. The apartment never smelled like smoke, the purifiers were always lit green, and his coughing fits stopped. So I thought I had done a decent job. For those other very brief instances of coughing, my brain always went to one of his other many conditions. Like I said, he had pacemaker surgery earlier this year (3rd degree AV block causing 30-70 focal seizures a day), and one of the coughs happened shortly after that. He had on and off very mild pleural effusion after his surgery. He had mild HCM. He had pretty severe arthritis. He had IBD/small cell lymphoma. He had so many things that seemed so much more serious at the time than his asthma, and I fucked up by focusing on those conditions and letting his asthma diagnosis go unmanaged because I thought I "fixed" the problem by reducing irritants. I was so overwhelmed by all of his supplements and medications that I didn't give him the one that could have really made him more comfortable. Moreso, I could have just stopped. Smoking. In. My. Fucking. Apartment. His asthma wasn't the cause of his death. He was diagnosed with granular large cell lymphoma in his intestine. That IBD/small cell lymphoma diagnosis quickly spiraled into an aggressive cancer. I had to let my baby go two days after his diagnosis. One thing that haunts me is that the week he passed away, he had an asthma flare up, just like the first one. Not one of the weird little single coughs that I worried could be related to his heart. We took him to the ER because his breathing seemed more labored one night. They xrayed him and said asthma flare up. We resumed the preventative inhaler he should have been getting the entire time. The next day, he had the low asthma posturing and a coughing fit. The day after that, he had abdominal swelling and we went back to the ER. Suspected mass in his intestine. Internal medicine appointment only 3 days after that. Cancer. Two days later, he's gone. In my arms, in his bed, looking into my eyes in the most peaceful way it could have went. They gave him Prednisone at the last vet appointment as a palliative care drug. I think it helped the asthma flare up too. I hadn't smoked in the bathroom for two weeks prior to this flare up happening, but I still blame myself. Because even if smoke didn't cause that one, giving him the fucking inhaler daily could have stopped it. I know his asthma didn't kill him, but it caused him discomfort while he was going through something even worse. And I could have at least taken that pain away. And I didn't. Despite his conditions, he was a happy, loving, playful, feisty guy. He only started to show signs of deteriorating in his last 2 months, starting with him simply not wanting to play with his wand toys anymore. He was a fighter, and he went through so much. I worried myself sick about him every day. I have so many videos on my phone of me recording his weird sleep breathing (and any instance of elevated breathing) out of concern. I don't know why my brain said "Pleural effusion, heart problems, etc." instead of "HE HAS ASTHMA BECAUSE YOU SMOKED IN THE ROOM WITH HIM FOR 6 YEARS. GIVE HIM HIS INHALER YOU MONSTER." I don't know what to do. I miss him so much. He was my everything. I talk to him every day and tell him how sorry I am. I feel like I don't deserve to grieve because I fucked up so bad. How could I do that to him for 6 years? How could I change my smoking setup to another (albeit less harmful) indoor setup instead of just GOING OUTSIDE? I'm afraid he's happier without me. I'll never be able to fix it. I'll never be able to really apologize. I'll never have the chance to change for him or know if he forgives me. I would do anything to go back and beat the shit out of myself. I genuinely hate hate hate myself, and I don't know how I'm supposed to go on for another 50 years with the weight of my filth. Without my baby boy. I'm so lost. And I'm so so sorry baby.