I am so sorry for what you're going through
I lost a close friend to suicide when I was 16 but I don't even think that's comparable to your situation. I've been trying to think about what helped me, but it's been such a long time. That is unfortunately most of what helps, time.
But journaling helped me, I think. Writing letters/messages to him (I mostly wrote to him on messenger (still do sometimes, like on his birthday)). Therapy.
Long-term though (and I hesitate to include this because I'm 99% sure it would Not have worked for me early on, mostly because I wouldn't have done/wanted to do it) a small but helpful thing is keeping a list/journal/calendar/whatever of The Good Stuff. In which every day you write down one good thing that happened.
I didn't start doing this to help with grief specifically, but I think it could still be helpful. Some days the good thing would just be "saw a dog from across the street". And on days where I couldn't think of a single good thing I would do something a little indulgent—draw a bit, watch a video I like, listen to a song, have a little treat, whatever—and write that down.
It trained my mind to look for good things, I think. It also encouraged me to do a little something for me on the tougher days.
After my friend passed I had trouble feeling joy at all for a while, so I don't know how useful this would have been for me at that specific time, and how useful it would be for you, but it's something I started doing later that genuinely helped me.
Again I am so very sorry for your loss. I can't fathom how you feel, but I hope this helps, even if only for the sentiment.
(also feel free to not respond to this, I saw one of your "older" posts asking for advice for dealing with incredible grief and wanted to at least try)
I actually really, really love the "good stuff" suggestion and will probably start doing that
thank you very much for that