Is It Me Or Is It Him??

In total despair I have been with my partner for 11 years he has constantly used however it has got worse over the years every birthday, holiday, christmas time he has this urge to go wild spend loads of money and go AWOL for a few days. He's tried everything but hooked on pure cocaine and ketamine when he can get it, I'm a social user so recently have been doing it with him as would rather so it with him than him go AWOL for days we also have 2 young children 9 and 2. It was my sons 2nd birthday and my partner had been taking legal highs the night before then dissapeared all morning, he came to the birthday meal but when everyone was to come back to mine for cake etc he turned on his mum which I thought was well out of order espesh infront if the kids, needless to say he fell out with me and I got accused of everything from going with friends to cheating on him years bk (all untrue) he kept reading my texts so I read his as wondered why so interested in mine and I found he's been trying to get with another girl, has a dating profile and has also told his friend he really doesn't like me. We never argue infront if the kids and I've done all I can to give them a good quality if life like my parents gave me, I work, I do all the things with the kids, pay the bills, holidays etc all because I thought it would get better I would have placed my life on it that he would never cheat. When confronted I was told I was boring and was good to speak to someone who liked him despite us actually in years with laughter with our banter the week previous (no drugs involved) and me taking him away for his birthday. It's coming up 2 weeks and no1 has seen him I have a close bond with his family and they are all worried and my older son is asking if he will see him again. I'm shell shocked, heart broken and basically dead inside, I also blame myself. Don't know what to do feel helpless over the whole situ when he's straight we have a great relationship but him being straight is becoming less and less followed by days sleeping writing this I know it's a bad relationship I just need some advice how to deal with this situ and understand. Did the drugs cos him to look elsewhere, how do I deal with my son, I just do not know what to do. Valentines day he wrote me such a special thing and gave me an eternity ring for Xmas but now he hates me and I feel I've fine nothing wrong?
Unfortunately you are in this situation and blaming the drugs isn't the answer. My relationship is not that long (almost 3 years, my last one was 27 years.) Thought we were everything to one another and have a six month old beautiful baby girl and everything possible to provide for her. I detached from her because of her pill abuse thought she was clean but she didn't tell me she was still getting prescriptions. We were apart for 7 weeks turned out she cheated on me with an old boyfriend she said was just a friend and I did not objected to it because I trusted her with that part of our life. Don't know why she lied about everything else..Really was crazy about this girl and keep letting her come back but she does not do anything to help me get over all she has done including losing custody of our daughter right after birth to her mother because of pill abuse in the hospital. I don't see any point nor do I see this ever getting better as hard as I've tried when she comes home I suggested earlier to get some of her stuff together and I will take her to her moms where she can be with our daughter and not be with me anymore because I believe she's doing things behind my backn She is no longer on the suboxone because of her teeth being pulled. Vicodin, back to oxy, then ? It's just going to go back to whatever. I don't know. I am heartbroken.I'm also stuck in a wheelchair for the next two months and get to see my daughter once a week. I blame the drugs for ruining my life with her but in reality I don't believe she wants this it's just a thing of convenience and I feel like I'm being used and as much as I hate being alone it's something I need to do to become a better person and be a better father