Oh, @Moonbelly . I have been exactly where you are just 2 years ago. My DD was almost 5 at the time.
I want to offer again to be there if you need a friend to message or call. You said your friends are great... but it's a unique circumstance and you might appreciate a stranger to chat to, but one who has been through the same.
You mentioned too your MIL made an awful comment about 'making it all about you'. Sadly I get this, too. The inlaws were awful. Full on denial at best, spite at worst. They still haven't actually told my DD her parent died! I found out totally by accident (I had finally been brave enough to split up a few months before exDP died).
I recall that feeling well of 'in the morning I need to tell my little girl her parent is dead'. It was the worst night I've ever had.
But I want to tell you that it WILL be ok. 2 years on my DD is thriving, and yours will too.
The death of an addict, and a young addict, brings about a very unique type of grief. I'm here if you need someone.
Please take care. In the next few days you may feel all over the place. My advice is keep hydrated, keep meals simple but keep eating. Accept offers of help. If friends say things like 'let me know if there's anything I can do' say yes! Say 'yes, I'd love if you could take my DD's to the cinema or playground for a couple of hours one night this week' - so that you can nap, or just have alone time with your thoughts (I found getting out for a slow, short walk helpful to clear my foggy brain).
Take care xx