I am really sorry you and your wife have been through all of that. Losing twins, watching your wife suffer physically, then trying to keep a big blended family going on top of your own mental health crashing, that is a huge amount for one person to carry.
I am glad you posted. That tells me there is still a part of you that wants connection, and that part matters. You are not weak for needing people. You are human. None of us are meant to get through grief, parenting, marriage pressure, suicidal thoughts and drug dependence alone.
I also want to say this gently. The fact that things got messy with dependence and attempts does not make you a bad person. It does mean you deserve proper support around you, not just willpower. A GP, your mental health team, a drug and alcohol support service, a counsellor, a peer group, even one safe person you can message when things get dark. Start building a small circle, one person at a time.
You sound like you love your wife and your kids, and you also sound exhausted. Being a dad and stepdad to six kids while grieving and trying to repair yourself is massive. Please do not try to be the strong silent one through this. Let people know plainly what you need. Something as simple as, “I am not doing great and I need someone to check in on me,” can be a real start.
I am happy to chat here. You do not have to have perfect words or pretend you are fine. Just keep talking.